Great　efforts　have　been　conducted　to　investigate　the　seismic　performances　of　the　arch　and　rectangular　underground　structures,　however,　the　differences　between　seismic　responses　of　these　two　types　of　underground　structures,　especially　the　vault　radian　influencing　the　seismic　responses　of　arch　structures　are　not　clarified.　This　paper　presents　a　detailed　numerical　investigation　on　the　seismic　responses　of　arch　underground　structures　with　different　vault　radians,　and　aims　to　illustrate　the　rule　that　vault　radian　affects　the　seismic　responses　of　underground　structures.　Five　arch　underground　structures　are　built　for　nonlinear　soil-structure　interaction　analysis.　The　internal　forces　of　the　structural　components　of　the　underground　structures　only　under　gravity　are　discussed　detailedly,　and　an　optimum　vault　radian　for　perfect　load-carrying　functionality　of　arch　underground　structures　is　suggested.　Then　the　structures　are　analyzed　under　seven　scaled　ground　motions,　amounting　to　a　total　of　35　dynamic　calculations.　The　numerical　results　show　that　the　vault　radian　can　have　beneficial　effects　on　the　seismic　response　of　the　arch　structure,　compared　to　the　rectangular　underground　structures,　causing　the　central　columns　to　suffer　smaller　axial　force　and　horizontal　deformation.　The　conclusions　provide　some　directive　suggestions　for　the　seismic　design　of　the　arch　underground　structures.