Fruit development is the key link in the reproductive cycle of plants, which determines the quality, yield and economic value of fruit. The present study investigated the effect of Sly-miR172a on the quality of the tomato fruit. The research results showed that overexpression of Sly-miR172a (OE-miR172a) in tomato fruit efficaciously increased the transverse and longitudinal diameter, weight, pericarp thickness, cell number and cell area. Compared with the wild-type (WT) fruit and the silencing miR172a by short tandem target mimic (STTM-miR172a), the relative expression levels of the MADS-box and the flavonoid biosynthesis synthesis pathway genes were higher in OE-miR172a. The expression level of AP2 and Cytokinin oxidase/dehydrogenase (CKX) family genes in OE-miR172a fruit was decreased, and accumulated more content of soluble sugars, lycopene, carotenoid, total phenol, anthocyanin and total flavonoid. The present study showed that overexpression of Sly-miR172 promoted the development and improved quality of tomato fruit.