In vitro metabolism study was conducted to observe the role of enzymes and its kind responsible for tolerance in metabolism of flupyrazofos to Diamond backmoth (DBM) , Plutella xylostella as a susceptible insect and Beet armyworm (BAW), Spodoptera exigua as a tolerance insect. The results indicated that there were some differences between the DBM ($LD_{50}$ 0.54 ppm) and BAW ($LD_{50}$ > 2,000 ppm). In the microsomal fractions, the 4th instar BAW larvae metabolized almost the same amount (about 38%) of flupyrazofos with or without NADPH, but it increased the amount of flupyrazol in the 3rd instar larvae of DBM. Flupyrazofos-oxon was slightly increased with the addition of NAUPH in both species. With the addition of piperonyl butoxide (PB) or Iprobenfos (IBP), the amount of flupyrazol was decreased to one third of flupyrazol produced by NADPH in BAW and one fourth of flupyrazol produced by NADPH in DBM. Some effects of inhibitors, PB and IBP, and co-factor, NADPH, ware showed when flupyrazofos was metabolized with or without them. This means that esterase and oxidase are responsible for the tolerance mechanism of flupyrazofos in DBM and BAW.