A green combustion technique assisted by freshly extracted tea (Camellia sinensis /natural tannic acid) was adopted to synthesize NiO nanoparticles. Further research into the physicochemical properties of the NiO nanoparticles was conducted after heating them to 350℃. The average particle size of NiO nanoparticles in their as-prepared state was found to be 18.4 nm, while the average particle size of NiO nanoparticles in their heat-treated condition was found to be 28.7 nm. The scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to record NiO nanoparticles' external morphology, particle shape, and particle size. It was discovered that the NiO nanoparticles had a spherical shape with an average particle size of 23.5 nm for the samples created as is and 34.8 nm for the heated samples. The performance assessment of the petrol sensor was carried out using a number of testing techniques and a variety of gases. NiO nanoparticles outperformed all other testing gases when it came to how effectively the sensor responded to ethanol gas. NiO nanoparticles are interesting materials for use in gas sensor applications, as evidenced by their response-recovery time.