To realize copper-based electrode materials for printed electronics applications, it is necessary to improve the adhesion strength between conductive lines and the substrate. Here, we report the preparation of Cu pastes using (3-glycidoxypropyl) trimethoxysilane (GPTMS) prepolymer as an adhesion promoter (AP). The Cu pastes were screen-printed on glass and polyimide (PI) substrates and sintered at high temperatures (> $250^{circ}C$) under a formic acid/$N_2$ environment. According to the adhesion strengths and electrical conductivities of the sintered Cu films, the optimized Cu paste was composed of 1.0 wt % GPTMS prepolymer, 83.6 wt % Cu powder and 15.4 wt % vehicle. After sintering at $400^{circ}C$ on a glass substrate and $275^{circ}C$ on a PI substrate, the Cu films showed the sheet resistances of $10.0m{Omega}/sq$. and $5.2m{Omega}/sq$., respectively. Furthermore, the sintered Cu films exhibit excellent adhesion properties according to the results of the ASTM-D3359 standard test.