Abstract: Tomato(Solanum lycopersicum) is the biggest economic crop in the world, the quality was extensively concerned by customers and scientists, and the fruit colors and firmness are important economic traits to determine tomato quality. To dissect the mechanism of fruit color development and firmness formation regulated by YFT1(YELLOW FRUIT TOMATO1), the differences of the fruit colors, firmness and microstructure were comparatively analyzed between mutant of yellow fruit tomato 1(yft1) and wild type(WT, cv.M82). The differences derived from fruit color, firmness and size and shape of pericarps cells were not found between yft1 and cv.M82 on the 35 days post anthesis(dpa). However, fruits turned into orange/pale red(47 dpa) and full red(54 dpa) from green(35 dpa) with fruit development, and fruit gradually turned into softening, and fruit firmness at 54 dpa(13.68±1.78 N) was only equal to one third at 35 dpa(35.51±1.09 N). Meanwhile, the size of pericarp cells was increasing with fruit development from 47 dpa, and from inner to outer. We also found that shape of the inner pericarp cell changed into oval(47 dpa) and irregular shape(54 dpa) from globular shape(35 dpa), moreover observed invagination of cells and disappearance of the space between cells on the 54 dpa. In contrast with cv.M82, no obvious differences were found for fruit color, firmness, size and shape of pericarp cells in mutant yft1 during 35 dpa to 54 dpa. However, the fruit firmness in yft1 tomato was significantly higher than that in cv. M82. The results implied that genetic lesion of YFT1 delayed tomato fruit development, thus influenced both of fruit color development and firmness formation. The present research would provide important evidence at phenotypes level to reveal the molecular mechanisms of formation of fruit color and firmness.