Abstract: To study the effect of temperature and photoperiod on the development of frost hardiness and to test whether the effect of both factors was additive, we carried out a growth chamber experiment for 2-year-old seedlings in Mongolia Scotch pine (Pinus sylvestris L. var. mongolica Litv.). There were four treatments involved in the trial: a long photoperiod (16h light: 8h dark cycle) with a high temperature (15℃)(LDHT), a long photoperiod (16h light: 8h dark cycle) with a low temperature (3℃)(LDLT), a short photoperiod (8h light: 16h dark cycle) with a high temperature (15℃)(SDHT), and a short photoperiod (8h light: 16h dark cycle) with a low temperature (3℃)(SDLT). Experiment duration was three months. Five tests were conducted at intervals of 2 or 3 weeks. Frost hardiness of needles was measured by the methods of electrolyte leakage (EL), visual scoring of damage (VSD) and chlorophyll fluorescence (CF) respectively every time. Results showed a stationary level of frost hardiness was reached in needles in a given temperature and photoperiod. Rate of frost hardening of needles differed among treatments, but the results did not support the additive concept. Frost hardiness of needles measured by these three methods was different. However, they had high correlations.