Oriental fruit moth (OFM), Grapholita molesta (Busck), is a key pest of tree fruit. Diapause plays an important role in overwintering larval of this pest, but which is still poorly understood. So the critical photoperiod for diapause induction in OFM and the age in day of larvae sensitive to diapause-inducting photoperiod were determined in the laboratory. The results indicated that OFM is a typical short-day type. The rate of larvae entered diapause>95% at photoperiod of 8L ∶ 16D, 10L ∶ 14D and 12L ∶ 12D, the critical photoperiod was 13.75L ∶ 10.25D at 20℃, the rate of larvae entered diapause>95% at photoperiod of 10L ∶ 14D and 12L ∶ 12D, the critical photoperiod was 13.68L ∶ 10.32D at 24℃. The age in different day of larvae show a degree of sensitivity when exposure to a diapause-inducing photoperiod, although there is no obvious critical point in time, but still the age in 5-8th day of larvae was more sensitive to diapause-inducting photoperiod, the percentage of diapause in larvae exposed to short photoperiod was signifiantly higher than that exposed to long photoperiod during this period.