Participatory　sensing　is　a　rising　paradigm　which　utilizes　mobile　phones　to　collect　data　and　build　application　on　the　cloud.　But　there　are　many　problems　to　be　resolved,　poor　quality　of　received　information　caused　by　task　executors　has　been　one　of　them.　So　incentive　mechanism　is　essential　for　attracting　users　to　participate　in　and　submit　high-quality　data.　Inspired　by　contract　theory,　we　model　participatory　sensing　as　a　contractual　relationship　and　devote　to　design　reasonable　rewards　for　relevant　results　to　maximize　the　benefit　of　task　publisher.　Under　complete　information　scenario　where　task　executors＇　efforts　can　be　observed　and　incomplete　information　scenario　where　task　executors＇　efforts　can　not　be　observed,　we　take　advantage　of　maximization　problem　to　infer　the　optional　contract　reward　for　task　executors.　In　addition,　based　on　the　utility　of　task　publisher,　we　propose　optimal　effort　and　optimal　effort　discriminant　inequality　(OEDI).　Furthermore,　we　discuss　the　influence　of　noise,　cost　and　boundary　on　optimal　effort　from　aspects　of　theory　and　reality.　Finally,　we　evaluate　our　contract-based　approach　by　thorough　simulations　to　show　its　effectiveness　and　accuracy.　©　2017　IEEE.