Thermal decomposition was conducted to investigate the influence of the various factors on stability of a new insecticide, [O, O-Diethyl O-(1-phenyl-3-trifluoromethyl-5-pyrazoyl) thiophosphoric acid ester : KH-502], in view of those informations applicable for industrial exploitation. In the thermal decomposition experiment, KH-502 was, after mixing with Fe, Cu and adjustment of moisture and pH conditions, subjected to three temperatures, 25, 50, and 100℃. Results for stability, and degradation pattern of KH-502 from the above experiment can be summarized as follows: 1. Main products of the thermal decomposition when this was conducted in the closed system were identified as following five compounds:O, O, O-Triethylthiophosphoric acid(TEPA), 1-Phenyl-3-trifluoromethyl-5-ethoxypyrazole(PTMEP), 1-Phenyl-2-ethyl-3-trifluoromethyl-5-hydroxypyrazole(PETMHP), O, O-Diethyl O-(1-phenyl-3-trifluoromethyl-5-pyrazoyl)phosphoric acid ester(KH-502 oxo form), O, S-Diethyl O-(1-phenyl-3-trifluoromethyl-5-pyrazoyl)phospho rothiolate(S-ethyl KH-502). However, compounds such as oxo form and S-ethyl KH-502 were not identified when the thermal decomposition was proceeded in the open system. 2. KH-502 was stable at 25 and 50℃, but it was decomposed at 100℃ following the first-order kinetics at the early stages of decomposition. 3. Rate constants for the thermal decomposition of KH-502 at 100℃ were in the orders of Cu powder addition 0.344$gt;Cu plate addition 0.21$gt;moisture addition 0.05$gt;closed system=open system=iron addition=pH 5.5 adjustment 0.04$gt;pH 8.5 adjustment 0.027 day^(-1), representing KH-502 was decomposed fast at Cu powder treatment and slow at pH 8.5 adjustment. 4. Half-life for the thermal decomposition of KH-502 at 100℃ was in the orders of Cu powder addition 2.02$lt;Cu plate addition 3.3$lt;moisture addition 14.5$lt;open system 16.3=pH 5.5 adjustment 16.3$lt;iron addition 17.3=closed system 17.3$lt;pH 8.5 adjustment 25.9 days. 5. KH-502 was converted into S-ethyl isomer and oxo form due to oxidation of P=S bond at the early stage of the thermal decomposition, and those intermediates were decomposed to PTMEP, PETMHP, and TEPA compounds continuously by cleavage of the molecular structure with time.