Due to the inherent dimensional uncertainty, tolerances accumulate in the final assembly. Tolerance accumulation has serious effect on the performance of ECV assembly. This paper proposes a method of tolerance accumulation analysis using Monte Carlo simulation, which includes welding process in assemble process. This method can predict the final tolerance distributions of the completed assembly with the prescribed statistical tolerance distribution of each part to be assembled. With the inclusion of welding, another dimensional uncertainties due to partial melting is to be accounted as well. Partial melting of projection height was included in the tolerance propagation analysis. Verification of the proposed method was performed by making use of Monte Carlo simulation. Monte Carlo simulation results showed promising results in that we can predict the final tolerance distributions in advance before actual assembly process of precision machinery.