Abstract: Using deltamethrin、phoxim and quinalphos to measure sensitivity of different instars cabbageworm( pieris rapae ). The bioassay results identified that the less poison capacity the more cabbageworm instar if the unit is pesticide weight of per cabbageworm larvae. From 1st to 3rd instar the more sensitive the more cabbageworm instar,the 3rd instar cabbageworm is more sensitive than 4th instar cabbageworm,the 5th instar cabbageworm is more sensitive than 4th instar cabbageworm if the unit is pesticide weight of per cabbageworm larvae weight.The experiment results identified that the 3rd instar cabbageworm larvae is the most sensitive instar. So the most important instar of cabbageworm for pesticide resistance monitory is 3rd instar,the most important pesticide control time is before 3rd instar.