Abstract: The quality difference of different size of Shatian pomelo fruit was studied and the mechanism of the quality difference was discussed in this paper. The Shatian pomelo was classified according to the fruit size,the soluble solid content,seed number,pericarp thickness and fruit index were measured,and transcription group sequencing was performed on the different sizes of Shatian pomelo. The results showed that,the moderate size of Shatian pomelo,i.e. 950 to 1000 g of Shatian pomelo had the highest soluble solid content,the thickness of Shatian pomelo peel and the number of seeds increased with the increase of the weight of Shatian pomelo. The fruit shape index was not affected by the size of Shatian pomelo fruit. After transcription group sequencing,compared with the KEGG database,20 pathways were significantly enriched,of which 10 pathways were related to flavor quality-related substance metabolism,2 pathways were related to plant hormones. The fruit size of Shatian pomelo was related to the content of soluble solids,the thickness of the peel,the number of seeds,while not related with the fruit shape index,and the difference in gene expression related to plant hormone signal transduction and flavor substance metabolism might be the cause of the quality difference.