Megalobrama pellegrini, a cyprinid fish belonging to Cultrinae, is an endemic fish of Yangtze river, lives only in the upper reaches of the Yangtze River basin, mainly in Sichuan Province. Due to the loss of habitat after the completion of the Three Gorges Project and other water-energy projects, it was forecasted that the population sizes of M. pellegrini might decreased sharply in the future. Taking suitable countermeasures for the conservation and resource-recovery of this endemic species becomes very urgent. Supported by the ecological and environmental monitoring network of the Three Gorges Project, studies on habitat, biology, reproductive ecology, artificial breeding and stocking of M. pellegrini have been making since 2001. The present paper deals with the reproductive biology of M pellegrini. 1382 specimen were collected during April 2001 to October 2003 from Longxi River, the first step branch of Yangtze River. The result shows that the spawning of M pellegrini population takes place from April to July, mainly in April and May. The sex-ratio of reproductive population of male Vs female is 1.83∶1. The maturing factor varies from 1.6%-42.1% of the matured females and 0.6%-11.5% of matured males. the average maturing factor of male shows just a peak in June during a year, meanwhile two peaks of the female were observed in a one-year cycle, which appears in April and July，and the crest is in July. The absolute fecundities of the matured femalesvaries are varied from 11011 to 249150, and 59587.22±59018 on average. The minimum size of matured male is 179.0mm in length and 57.1g in weight; while that of matured female is 189.0mm in length and 72.1g in weight, both of them are 2 year-old. Statistical analysis of the maturing condition shows that nearly 75% of the female and 94.7% of the male were matured in the 2-year group; all of the 3-year group were matured. The breeding population mainly consists by 2-age and 3-age group (74.6%), which means the supplementary population dominates the breeding population.