Larvae of Bombyx mori fed on the mulberry leaves treated with pyriproxyfen extended the larval duration. The silkworm on the third day of the fourth instar fed pyriproxyfen 10‐6 concentration extended the fifth instar duration over 20 days and was not able to spinning silk. Pyriproxyfen was treated at the third day of the fourth instar, the first day of the fifth instar and the fifth day of the fifth instar larvae, the larval body weights were higher by 49%, 38%, 44% than those of the control, respectively. Pyriproxyfen was treated at the third day of the fourth instar, the first day of the fifth instar and the fifth day of the fifth instar larvae, pupal ratio was 91.3%, 80.0%, 12.0% of the control, respectively. Pyriproxyfen was treated at the third day of the fourth instar, the first day of the fifth instar and the fifth day of the fifth instar larvae, the single cocoon weights were 2.11g, 2.99g, 3.83g and cocoon shell weight was 43.2cg 40.9g 20.1g, respectively. When pyriproxyfen was treated at the third day of the fourth instar, the first day of the fifth instar and the fifth day of the fifth instar larvae, the maximum fifth larval silk gland weight of the fifth instar treatment was lower by 59%, 54% than those of the control.