To analyse growth and variation in trend of some traits of potato a field experiment was carried out in 2003 and 2004 at field research station of Seed and Plant Improvement Institute, Karaj, Iran. A split plot arrangement in randomized complete block design with three replications was used. Planting dates at five levels; 08 April, 24 April, 09 May, 25 May and 09 June were assigned to main-plots and three potato cultivars were randomized in sub-plots. Combined analysis of variancse revealed significant differences among planting dates and potato cultivars for most of the studied traits. Response of potato cultivars to planting dates was different considering the slopes of variation in trends of different traits. Therefore, the observed response are specific to each cultivar. Planting date significantly affected yield and yield components of cv. Agria and variation in trend of traits differed with planting dates. Variation in trend of tuber dry matter (%) and yield showed that both were at maximum level when accumulated growing degree days reached 1000 ºCd after flowering. Variation in tuber growth rate, tuber relative growth rate and tuber yield in different planting dates indicated that there were in agreement, and these indices and tuber yield changed in the same direction in different planting dates. Tuber growth rate varied from 1.2-1.5 g m-2oCd-1 and the maximum tuber relative growth rate was 0.125 g g-1oCd. It was concluded that the third (09 May) and fifth (09 June) planting dates are suitable for cv. Agria in Karaj region.