An experimental study was conducted to investigate the effect of pulsating Strouhal number, Sγp, on flow and heat transfer around a heated cylinder in a pulsating cross-flow. Averaged Reynolds number was kept at 400 while oscillatory Reynolds number was changed from 40 to 300. Pulsating Strouhal number, Sγp, ranged from 0 to 1.37. Two remarkable peaks of the mean Nusselt number were observed at approximately Sγp=0.2 and 0.6. Flow visualization experiments revealed that twin symmetric vorticies were periodically formed behind the cylinder at Sγp corresponding to the 'lock-on' of the shedding of vortices. From results of velocity field measured by PIV, it was found that the flow reversal approaching to the back face of the cylinder was intensified by twin vortices. This flow reversal caused heat transfer enhancement on the back face of the cylinder.