To　reduce　the　position　error　of　a　parallel　robot　caused　by　joint　clearances,　a　method　to　compensate　the　joint　clearance　errors　was　proposed　by　optimizing　the　joint　angular　displacement　parameters　of　drivers.　For　the　purpose　of　a　planar　3-RRR　parallel　robot　system,　an　error　model　of　the　joint　was　presented　and　the　change　rules　of　the　errors　of　actual　lengths　of　links　and　actual　joint　angular　displacements　of　drivers　caused　by　joint　clearances　were　obtained.　According　to　the　inverse　kinematics　equation,　the　error　analysis　model　of　system　was　proposed　by　total　derivative　theory.　The　structural　errors　of　system　caused　by　joint　clearance　errors　were　compensated　by　optimizing　the　joint　angular　displacements　of　drivers　using　Particle　Swarm　Optimization　(PSO)　algorithm.　By　inducing　the　linear　decreasing　weight　and　the　compression　factor,　the　standard　PSO　algorithm　was　improved,　and　an　unified　expression　of　algorithms　was　derived.　The　results　of　error　compensation　show　that　the　improved　PSO　algorithm　is　effective　for　improving　the　convergence　performance.　The　trajectory　errors　of　end-effecter　using　optimized　joint　angular　displacements　are　decreased　by　99%　as　compared　with　that　of　uncompensated　trajectory.　Obtained　results　demonstrate　that　the　proposed　method　effectively　compensates　the　structural　errors　caused　by　joint　clearances　and　guarantees　the　position　accuracy　of　parallel　robot　systems.　©,　2015,　Chinese　Academy　of　Sciences.　All　right　reserved.