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.