Abstract: Three sites (high, medium and low altitude) were established in an alpine mountainous region in the eastern Tibetan Plateau. The reproductive organs of Fritillaria unibracteata at each site were measured. A transplant experiment using F. unibracteata was also carried out; plants from low altitude were transferred to the high site and plants from the high site transferred to the low altitude site. Ecological factors such as snow-melt duration, air temperature, soil water content and soil pH were measured. Reproductive organs including flower, fruit and seed of F. unibracteata were also measured. Snow-melt duration, air temperature and soil water content differed among the different altitudinal sites significantly. Snowpack at the low site melted earlier than those at the other two sites; average air temperature (in March, April and May) at the low site was higher than the other two sites; however soil water content at the low site was the lowest. The highest values for petal length and width, filaments, anthers, styles and style ramifications were measured at the high site and the least at the low site. Flower biomass at the high site was 39.4% heavier than that at the low site, and the ratio of flower biomass to aboveground biomass at the high site was 12.3% higher than that at the low site. Dry weight per fruit at the low site was 48.3% heavier than that at the high site, reaching 1282 mg. Seed number per fruit at the high site was the lowest, 38.1% less than that at the low site. The 100-seed weight at the high site was 142.3 mg, 94.6 mg at the low site. The transplant experiment verified the results described above. We concluded that the reproductive organs of F. unibracteata were significantly affected by altitude factors, particularly snow-melt duration, air temperature (especially sub-zero temperatures), ultraviolet light and insect pollinators.