Zirconium base alloys are used as critical components of water cooled nuclear power reactors. The phase transformation and microstructural evolution in these alloys are complex. Depending on soaking temperature and cooling rate, the β phase of these alloys can transform into a variety of microstructures, viz., allotriomorph alpha, Widmanstatten alpha with parallel-plates or basket-weave morphology, martensitic microstructure, and omega phase. An advanced numerical modeling technique, the “Phase field method” (PFM) has been used to study morphological evolution of β–Zr phase in dilute Zr–Nb alloys during β – Zr (BCC) → α – Zr (HCP) transformation in mesoscopic scale. The nucleation events have been incorporated in the model both explicitly and implicitly. The growth rate and morphology selection have been investigated by varying the supersaturation, i.e., temperature and Nb content in Zr–Nb alloys. For Zr–2.5 %Nb with low undercooling, the driving force for plate growth decreases and at 1054 K it shows formation of allotriomorph α through classical diffusional transformation. Higher undercooling favours Widmanstatten lath formation. The growth velocity of the Widmanstatten plate is constant. The change in alloy composition changes the effective supersaturation at the α/β interface and leads to a different tip velocity. When Nb content reduces below 200 ppm, lath morphology does not develop and the interface remains planar only.