Formaldehyde (formalin) is a preservative that is widely used in surfactants, dishwashing liquids, cosmetics and health products, especially shampoos and care products. The International Agency for Research on Cancer has identified formaldehyde as a strong carcinogen. Therefore, the detection and measurement of this compound with high speed and sensitivity in various samples is of great importance. Various methods for measuring formalin have been reported in scientific literature, but they have not been accepted due to long analysis time, high cost, and low sensitivity. Among the methods, electrochemical methods have been considered due to their high sensitivity, high analysis speed, low cost, and miniaturization. In this study, the surface of the pencil lead electrode was graphitized by applying a direct voltage in a sulfuric acid solution. After activating the electrode surface, it was floated in nickel chloride solution and nickel nanoparticles were deposited on the electrode surface using chronoamperometry technique by applying a potential of -1.2 for 100 seconds. Then, by scanning the potential in alkaline medium, the graphenized pencil lead electrode (GPLE) modified with nickel hydroxide nanoparticles was obtained. The morphology of the GPLE/Ni(OH)2 was examined by scanning electron microscopy and the average particle size was calculated to be about 52 nm. The structure of the modified electrode was characterized using X-ray diffraction (XRD), electrochemical impedance spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM) and electrochemical methods. The important parameters including the electrodeposition time, electrodeposition potential and electrode activation conditions were optimized. After electrochemical experiments, the prepared electrode was used to measure formaldehyde as a hazardous environmental pollutant in various types of detergents. Based on the experimental results, the simultaneous presence of nickel hydroxide nanoparticles and graphene nanosheets on the electrode surface increases the electron transfer rate and also increases the electrochemical active surface area of the modified electrode. The results of the experiments conducted under optimal conditions show that the GPLE/Ni(OH)2 has a linear range of 5-175 μM, a detection limit of 2.04 μM, and a sensitivity of 0.20496 μA μM-1 cm-1 for the measurement of formaldehyde. Compared to similar studies, the efficiency of the GPLE/Ni(OH)2 in the measurement of formaldehyde is acceptable and it can be used with high accuracy and reliability in the measurement of formaldehyde in various detergents. In order to investigate the effect of interference of some species on the response of the fabricated sensor, the interference of compounds such as betaine, coconutate, texapone, urea and glycerin, which are widely used in detergents, was investigated. According to this study, the change in the response of the GPLE/Ni(OH)2 electrode for 25 μM formaldehyde in the presence of interfering species was less than 5%, indicating good selectivity of the designed sensor. Also, the fabricated electrode maintained its efficiency for long periods of time, which indicates its commercialization. The results obtained from the spiking and the recoveries obtained indicate the high accuracy of the results obtained. The results obtained using the proposed method were compared with some other methods which indicated the high sensitivity and capability of GPLE/Ni(OH)2 for measuring formaldehyde with high accuracy and precision.