Mechanical stresses which develops during lithiation of crystalline silicon particles in lithium silicon battery causes fracture and limits the life of silicon based lithium batteries. We formulated an elasto-plastic stress formulation for a two-phase silicon model and investigated the influence of different mechanical properties of lithiated silicon on the fracture of nanoparticles during first cycle charging. A chemo-mechanical model was developed to determine lithium distribution and associated stress states during first cycle lithiation. The concentration gradient of lithium and an elastic perfectly plastic material behavior for silicon were considered to evaluate stress distribution formulation and determine stress field in the particle. The stress profile was used to perform a crack growth analysis. The stress distribution formulation was validated by evaluating stress field for different elastic modulus value for lithiated silicon and comparing our inference against observations from prior experiments. The results showed lower modulus of lithiated silicon yielded results like experimental observations for nanoparticles. The size dependent fracture behavior was also observed in lower elastic modulus of lithiated silicon. We conclude that accurate mechanical characterization of lithiated silicon nanoparticle is necessary to model the failure of silicon particle and improving the mechanical properties may suppress crack growth in silicon nanoparticles during charging.