Composing　distributed　trivial　grid　services　into　composite　and　more　powerful　services　is　an　effective　way　to　realize　grid　resource　sharing　and　function　reuse.　BPEL4Grid　is　a　grid-compliant　workflow　engine　which　supports　for　WSRF　standard　and　GSI　mechanism.　But　BPEL4Grid　has　some　limitations　such　as　its　static　workflow　model　and　lack　of　task　migration　when　target　service　is　invalid.　Aiming　to　resolve　previous　problem,　the　extension　of　BPEL4Grid　which　supports　abstract　process　definition　is　proposed.　Base　on　the　extended　BPEL4Grid,　an　improvement　to　ActiveBPEL　was　presented.　A　QoS-based　service　evaluation　model　is　described.　The　experiment　indicates　that　the　system　is　more　adaptive　with　grid＇s　dynamic　characteristic.　©　2011　IEEE.