Abstract： The Martensitic (MP) and incommensurate/commensurate (I/C) phase transformations during the athermal process of NiTi alloy were studied by the measurements of internal friction, elastic modulus and electric resistance simultaneously. The IF behaviour in the processes of MP and I/C phase transformations are nearly the same and is associated with the motion of phase interface under the action of phase transformation driving force. Starting with the interface dynamics model, the interface dynamic behaviour of MP and I/C transformations were . studied. An explicit functional relationship of the interface dynamics in the process of marten-sitic transformation was obtained as V = V* exp(-△G*/△G - △GR) and the expression for IF during the process of phase transformation as Q-1 = (n2/2)×(μ△G*/(△G-△GR)2) dF/dT·T/ω. The transformation resistance froce △GR is the order of 10 cal/mol. The "soft mode effect" during the transformation process was discussed, and it was shown that the softfening of pho-non mode and the modulus defect associated with interface motion coexist in the processes of martensitic and I/G phase transformations.