Ultrafast spin transfer torque (STT) switching in an in-plane MgO magnetic tunnel junction with 50nm×150nm elliptical shape was demonstrated in this paper. Switching speeds as short as 165ps and 190ps at 50% and 98% switching probabilities, respectively, were observed without external field assistance in a thermally stable junction with a 101% tunnelling magnetoresistance ratio. The minimum writing energy of P-AP switching for 50% and 98% switching probability are 0.16pJ and 0.21pJ, respectively. The observed ultrafast switching is believed to occur because of partially cancelled out-of-plane demagnetizing field in the free layer from interface perpendicular anisotropy between the MgO layer and the Co 20Fe 60B 20 layer. High J/J c0 ratio and magnetization nucleation at the edge of free layer, which result from the reduced perpendicular demagnetizing field, are possibly two major factors that contribute to the ultrafast STT switching.