We have measured temperature dependence of the uniaxial magnetocrystalline anisotropy constant K-u and the shear stress tau(req) required for the twinning plane movement of an Fe-31.2Pd alloy in order to explain the field orientation dependence of rearrangement of martensite variants (RMV) of this alloy in the field-cooling process. Under the [001](P) field (P represents parent phase), the RMV occurs perfectly at the temperatures where the measurement was performed; in this case, the magnetic shear stress acting across the twinning plane tau(mag), which is evaluated from K-u, becomes nearly twice as large as tau(req). On the other hand, under the [011](P) field, the RMV occurs partially at all the measuring temperatures; in this case the maximum of tau(mag) is nearly the same as tau(req). It is successful to evaluate the following relation quantitatively; tau(mag)>tau(req) is satisfied when RMV occurs under magnetic field.