Basing on fruit size difference of ‘Bingtang’ sweet orange(Citrus sinensis (L.) Osbeck) planting in Hongjiang City, Hunan Province and Huaning County, Yunnan Province, the extrinsic qualities and regional climate were comparatively analyzed. The results showed that the fresh fruit mass was 147.56 to 159.80 g/fruit in Huaning, which was significantly bigger than that of 106.68 to 109.41 g/fruit in Hongjiang. Along with the bigger fresh fruit mass, fruit diameter of Huaning was significantly larger. Further analysis revealed that amounts of both fruit segments and juice sacs in one segment exhibited no significant difference between the two areas. However, the single juice sac volume of Huaning ‘Bingtang’ sweet orange was 22.07 mm3, significantly bigger than 16.62 mm3 of Hongjiang ‘Bingtang’ sweet orange. Comparison of environmental conditions showed that there was no significant difference in the annually average temperature and annually accumulated temperature between the two areas. The annual rainfall in Huaihua area reached 1450.3 mm, with heavy rainfall from April to middle July when was at the end stage of fruit cell division and the early stage of fruit enlargement. That resulted in a water use efficiency of approximate 1.23 kg/m3. Yuxi area had an annual rainfall of 1032.1 mm and greater rainfall from July to September when the fruit was at enlargement stage with a water use efficiency of approximately 3.78 kg/(667 m2)?mm. Additionally, the overall phenological period was about a month earlier in Huaning than that in Hongjiang, and the fruit developing period was also about a month longer than that of Hongjiang. In conclusion, three reasons, the stabler annual temperature, the longer shoot and fruit development periods and the increased juice sacs size with adequate water supply at enlargement stage were closely related to the bigger fruit size of ‘Bingtang’ sweet orange in Huaning area.