This research was carried in cropping years 2017-18 and 2018-19 to study the effect of plant density on morphological traits, flowering pattern, boll production and seed cotton yield of new cotton cultivars in Varamin region of Iran. In this research, five cotton cultivars including Varamin, Shirpan603, Cocker100, NO200 and SP731 in three plant densities were evaluated using a factorial experiment in a randomized complete block design with three replications. In this study, the number of flowers and flowering date in cultivars for 11 weeks (1 July to 15 September) were counted. The results of analysis of variance showed that the differences between cultivars were significant for plant height, number of nodes, number of flowers, number of bolls and fiber and seed cotton yield at 1% level. The results also showed that there is a significant difference between the years of project implementation for plant height, number of bolls and fiber and seed cotton yield at the level of 1%. The mean of plant height for the first and second years were 93.2 and 119.9 cm and the average number of bolls per plant were 15.9 and 11.41 and the seed cotton yield were 5242 and 4050 kg/ha, respectively. In this study cultivars did not differ significantly in terms of number of days from planting to flowering, number of days from flowering to harvest and early ripening. Investigation of flowering process in 11 weeks showed that the peak of flowering for both years of research was the eleventh week after planting with an average of 41.8 and 44.6 flowers per plant. Most of the bolls were produced from the flowers of the eleventh week for first year and from the flowers of the tenth week for second year. The cumulative growing degree days during the vegetative and reproductive growth period for two years of experiment was significantly different.  So that at the end of the seventeenth week after planting (last week of flowering) the cumulative growing degree days in the first and second years was 1734.9 and 1245.5, respectively, Which can be one of the important reasons in reducing the number of bolls and fiber and seed cotton yield in the second year.