The　analysis　of　fatigue　property　was　incorporated　into　the　research　on　dynamic　stress　of　flexible　robots.　The　relation　between　elastic　deformations　and　elastic　displacements　of　the　flexible　planar　robot　was　investigated.　The　elastic　deformations　of　links　were　calculated.　The　dynamic　stress　on　any　position　of　the　links　was　derived.　The　fatigue　failure　due　to　the　alternate　dynamic　stresses　was　analysed.　The　fatigue　lives　of　the　links　were　predicted　based　on　the　cumulative　damage　rule.　The　importance　of　dynamic　stress　analysis　was　well　illustrated　through　the　numerical　simulation　of　a　flexible　planar　3-RRR　parallel　robot.　The　dynamic　stress　analysis　is　demonstrated　to　have　significant　impact　on　the　design　and　control　of　flexible　robots.　©2006　IEEE.