The effectiveness of residual stress on forming copper patterns of printed circuit board was investigated during applied thermal conditions. Generally, the electrolytic copper foil showed a compressive residual stress about -54MPa as received, which easily caused to form copper patterns irregularly. We verified the compressive residual stress was relaxed with applying thermal conditions under 200°C for a few hours. And also, we observed that the compressive residual stress of copper foil tended to be relaxed, constant, and compressive again during heating times at each temperature. The relationships between residual stress and etching factor of copper pattern were analyzed in this works.