Diffusion-induced stresses in Sn, a promising anode material for Li-ion batteries owing to its high specific capacity, depend significantly on the phase transformation mechanism. In this study, an in-situ X-ray diffraction study is performed to reveal the phase transformation mechanism in Sn as functions of the discharge rate and Sn anode dimensions. In a 500 nm-thick Sn thin-film discharged at C/9 or a 100 nm-thick Sn thin-film discharged at 0.1 C, the Sn phase transforms sequentially to Li2Sn5, followed by β-LiSn and a-Li7Sn3 in three steps, where each step involves reaction-controlled lithiation. However, in a 500 nm-thick Sn thin-film discharged at 2 C or a 2 μm-thick Sn thin-film discharged at 0.1 C, the a-Li7Sn3 phase is directly formed via one-step reaction-controlled lithiation between Sn and a-Li7Sn3. A transition from three-step to one-step results in a steep gradient in the mismatch strain, thereby causing early failure. Finite element simulations show a lower J-integral for the three steps compared with that of a one-step reaction, thereby confirming previously reported experimental observations. For a specified transformation mechanism, the J-integral is lower for smaller Sn micropillars. Therefore, the mechanical reliability of the Sn anode can be enhanced significantly when lithiated under phase transformation mechanism involving three-reaction-controlled lithiations, as well as utilizing a small Sn anode measuring less than 200 nm.