A new diamine was designed and synthesized to improve the flexibility of colorless polyimides by reducing residual stress. Four variations of colorless polyimides with the same dianhydride (4,4' (hexafluoroisopropylidene)- diphthalic) and four different diamines (bis[4-(3-aminophenoxy)-phenyl] sulfone, bis(3-aminophenyl) sulfone, 2,2'- bis(trifluoromethyl)benzidine, and 2,2-bis(4-aminophenyl)-hexafluoropropane) were used. A series of colorless polyimides were prepared by adding the new diamine. The carbon and ether bonds between the benzene rings of the new diamine affected the flexibility and optical properties of colorless polyimide. The synthesis of the new diamine was confirmed by NMR measurements. Furthermore, the decrease in residual stress at room temperature and the glass transition temperature was confirmed. The effect of the new diamine was most evident for polyimide with a bulky and rigid structure. Though a slight yellow color appears because of the broken charge transfer complex balance, controlling the content of the new diamine will allow application of polyimides in flexible display.