Abstract: Trust is introduced into grid service composition based on group dynamic trust model.Thus component grid services with higher trust degree can join in the grid service composition procedure and the trust degree of the composite grid service can be enhanced.Finally,user can get more trusted services.Trust degree estimate problem of activities in grid service composition model is discussed and corresponding estimate formulae are proposed.A trust degree estimate algorithm of composite grid service is designed and the complex degree is analyzed.This algorithm is valid through complex degree analysis.