Crowdsourcing　is　a　way　of　collecting　adequate　and　valuable　data　from　a　group　of　users　with　sensors-embedded-in　mobile　devices.　Because　of　the　information　asymmetry　between　crowdsourcer　and　users,　one　problem　called　moral　hazard　may　arise,　which　is　the　crowdsourcer　has　no　idea　about　whether　the　users　will　provide　qualified　data　and　how　much　effort　exerted　by　them.　Accordingly,　it　is　necessary　to　introduce　contract-based　incentive　mechanism　to　provide　participants　with　a　certain　reward　to　overcome　the　moral　hazard　problem,　so　as　to　achieve　the　purpose　of　encouraging　more　users　to　participate　data　collection.　A　novel　contract　is　proposed　to　model　one　crowdsourcer　and　multiple　users　engaged　in　exchange　for　data　and　money,　in　which　the　users　are　of　different　relationships,　which　are　cooperative,　competitive,　and　independent.　Then,　we　derive　the　optimal　contract　that　maximizes　the　crowdsourcer＇s　utility　while　achieving　the　benefits　of　the　users.　Finally,　through　simulations,　we　reveal　the　key　parameters＇　influences　on　the　crowdsourcer＇s　utility　under　three　relationships,　and　shows　the　proposed　contracts　are　efficient　and　effective.　©　2019　IEEE.