This　paper　aims　at　analyzing　the　influence　of　non-associated　flow　rule　on　face　instability　of　a　circular　tunnel　in　cohesive-frictional　soils.　A　series　of　three-dimensional　numerical　simulations　are　performed　to　analyze　the　influence　of　dilatation　angle　on　the　limit　support　pressures　and　the　failure　zones.　The　dilatation　angle　significantly　influences　the　limit　support　pressure　and　the　failure　zone.　Then,　a　three-dimensional　failure　mechanism　that　consists　of　the　ellipsoid　and　the　multi-block　is　proposed　based　on　the　upper-bound　theorem.　Finally,　the　limit　support　pressures　and　failure　zones　obtained　from　the　new　mechanism　are　compared　with　those　given　by　existing　approaches　of　limit　analyses,　numerical　simulation　and　centrifuge　tests.　The　results　indicate　that　the　presented　model　is　more　effective　in　analyzing　the　face　stability　of　tunnels.