Abstract: In order to make a scientific and practical plan for conservation of an endangered species, it is indis-pensable to identify the kind of insects effective in legitimate pollination and to clarify the main factors on their pollination behavior. Primula merrilliana, a small biennial rosette herb with distylous flowers, is an endangered and endemic species of China. In this study, the effective pollinators and the effects of flower density and popula-tion size on pollination efficiency were investigated. The results indicated that the primary effective pollinators of this distylous herb were two bee flies, Bombylius major and Anastoechus chinensis rather than Thrips sp., which was previously considered as an effective pollinator for the plant. The density of flowers and the population size were the two main factors that influenced pollination efficiency of P. merrilliana. Flowers with higher density attracted more pollinator visits, particularly in low dense populations. The number of pollinators and flowers visited per day and the average time of a flower visited per day were significantly positively correlated to the density of flowers in the samples. Legitimate pollination and its percentage of total stigmatic pollen loads were higher in larger populations. The biased ratio of short-styled and long-styled flowers and pollinator limitation might be responsible for pollination disadvantages in small populations.