This paper reports high performance flexible lamb wave sensors with a graphene oxide sensing layer. The devices were fabricated on piezoelectric ZnO thin films deposited on flexible polyimide substrates. Two resonant peaks, namely the zero order antisymmetric (A0) and symmetric (S0) mode lamb waves, were observed and fitted well with the theoretical analysis and modelling. With graphene oxide microflakes as the sensing layer, the sensing performance of both wave modes was investigated. The humidity sensitivity of the A0 mode is 145.83 ppm/%RH (at humidity 85%RH), higher than that of S0 mode of 89.35 ppm/%RH. For the first time, we demonstrated that the flexible humidity sensors work as usual without noticeable deterioration in performance even under severe bending conditions up to 1500 . Also the sensors showed an excellent stability upon repeated bending for thousand times. All the results demonstrate that the lamb wave flexible humidity sensors have a great potential for application in flexible electronics.