The sequential growth of lath martensite was clarified using a high temperature confocal scanning microscopy by intentionally introducing gradient in martensite start temperature in ultra-low carbon Ni steels. The plastic accommodation in austenite during the formation of lath martensite was further evaluated at the growth front of lath martensite using Wilkinso method, and the result are compared with the phase filed model incorporating crystal plasticity. The shape change accompanying martensitic transformation is accommodated by plastic deformation in either austenite matrix or martensite. Even though the accommodation behavior appears to affect the sequential formation of lath martensite, it has not been thoroughly investigated because of the difficulty in capturing growth front of lath martensite. In this study, 23 mass% and 36 mass% Ni ultra-low carbon steels were pressure bonded at 1200 K, and then held for 240 h to induce Ni gradient in the vicinity of the initial bonding interface.The variant selection during the sequential formation of lath martensite will be further discussed.