Optimal　path　planning　and　trajectory　tracking　of　underactuated　robots　are　investigated.　The　control　target　is　to　realize　the　arbitrary　position　control　of　joint　angles.　The　controllability　of　underactuated　robots　is　analyzed.　The　thought　of　partly　stable　controllers　is　introduced　and　the　best　switching　sequence　of　partly　stable　controllers　is　obtained　by　using　genetic　algorithm.　Sliding　mode　control　method　is　utilized　to　realized　accurate　model　tracking.　The　proposed　method　has　following　advantages:　It　does　not　make　any　hypothesis　about　the　rigorous　linearizations　to　the　original　nonlinear　system.　Also　because　of　the　global　search　ability　of　genetic　algorithm,　the　optimal　trajectory　can　be　generated　quickly　and　accurately.　The　accurate　trajectory　tracking　can　be　guaranteed　by　the　good　adaptability　to　the　disturbance　and　parameter　changing　of　sliding　mode　control　method.　Therefore　the　proposed　method　can　be　used　without　modification　for　arms　with　a　large　number　of　degree　of　freedoms.　At　last　numerical　simulations　are　carried　out　on　the　planar　3R　underactuated　robot　to　verify　the　validity　of　the　proposed　method.　©　2009　Journal　of　Mechanical　Engineering.