The internal friction (IF) Q-1, modulus f2, electrical resistance R and stress s of superelastic Ni51Ti49 alloy in the process of stress induced incommensurate/commensurate (I/C) transition was measured as a function of strain e by a middle torsion pendulum on a tensile testing machihe modified with a four-terminal potential equipment.The effects of number of stress cycles and strain rate e (in the range of 7×10-7/s to 1×10-4/s) were studied. An IF peak and a minimum of modulus were observed respectively in the Q-1-εand f2-εcurves. The IF peak height Qp-1 and modulus defect △M/M increase with an increasing of ε/ω, where ω is angular frequency of measuring. By utilizing an expression obtained from phase interface dynamics and experimental data of Qp-1 (as a function of ε/ω and amount of phase transition F), an explicit functional relationship of average I/C phase interface velocity V and effective phase transition driving force △G′= △G - △GR (where △GR is resistance) was derived as V =V*(△G - △CR)m, where V* and m are dynamic parameters an △GR is a resistance force exerting on the moving interface, which has the order of 1 cal/mol for stress-induced I/C transition of superelastisity NiTi alloy. The relationship between Q-1 and modulus defect △M/M and soft modulus effect are disussed.