We present our study on B → pi;lν semileptonic decay form factors using NRQCD action for heavy quark. In the analysis, we use the form factors f1(·kπ) and f2(·kπ) defined in the context of heavy quark effective theory by Burdman et al.. Since the NRQCD action for heavy quark respects the heavy quark symmetry, our results are described by the HQET form factors most naturally. From a quenched lattice QCD simulation at β=5.9 on a 163 × 48 lattice, we obtain the form factors f1(·kπ) and f2(·kπ), and find that their 1/mB correction is small. The limit of physical heavy and light quark masses can be reached without introducing any model function, and we obtain a prediction for the differential decay rate dΓ/dq2. We also discuss the soft pion limit of the form factors.