Nickel phthalocyanine (NiPc) is one of the most promising organic semiconductor having potential applications in photovoltaic and gas sensing technologies because, they have high absorption coefficient in the wide spectrum and high photo-electromagnetic sensitivity at low intensities of radiation. In this study we have fabricated NiPc based field effect transistors (FETs) of varying channel thickness and investigated the effects of heating on their electrical characteristics. NiPc and semi-transparent aluminum (Al) thin films were deposited in sequence by vacuum evaporation on a pre-deposited silver (Ag) drain-source electrodes. The flow of current was controlled using Schottky junction defined by metal (Al) semiconductor (NiPc) interface. At 100 Hz operational frequency, it is observed that by increasing the ambient temperature from 26-62 ^oC, the impedance of the transistors decreases. It is observed that for 100 nm, 200 nm and 300 nm thick films FETs, the reduction in the impedance was 2.3, 1.5 and 1.3 times, respectively. Temperature coefficients for impedance change were also evaluated and found to be -2.35 % / ^oC, -1.39 % / ^oC and -1.18 % / ^oC for 100 nm, 200 nm and 300 nm thick films FETs, respectively. Moreover, impedance-temperature relationships obtained for NiPc based FETs were also simulated and discussed.