The effect of temperature (15, 25, 30°C and glasshouse as control) at different growth stages (stage-I: vegetative to tuber initiation, stage-II: tuber initiation to initial tuber bulking and stage-III: tuber bulking to maturity) on leaf nitrate reductase (NR) activity, carbohydrate contents and tuber yield were investigated in potatoes (Solanum tuberosum L. cvs. May Queen and Normn 1) as pot experiment in naturally lit glasshouse and phytotron. The high temperature treatment decreased the total dry matter production as well as tuber yield and degraded the tuber quality by reducing the specific gravity. NR activity of leaves was also decreased by high temperature treatment, whereas low temperature increased the activity. Water soluble carbohydrate (WSC) and starch content in leaves were sequentially decreased and as a result total nonstructural carbohydrates (TNC) were also decreased by high temperature. The inhibition of tuber yield by high temperature was due to the limited translocation of carbohydrates from leaves to the tubers, following the reduction of NR activity and the expense of carbohydrates for dark respiration. The maximum yield reduction was observed at high temperature treatment at stage-I which was most critical for tuber yield, and low temperature at stage-II produced the highest yield which was considered as more advantageous step towards yield improvement.