A direct Monte Carlo Method is used to simulate the effect of tri-and tetra-functional branching on molecular weight distribution in emulsion polymerization of butadiene. Butadiene polymerization due to high extent of transfer to C=C bonds of polymer chains, can be used as a modeL to study the effect of tri-and tetra-functional branching on Polymer Microstructure. In this simulation, elementary reactions included propagation, chain transfer to Monomer, termination by disproportionation, transfer to C-H bonds (BN3) and C=C bonds (BN4) of growing and dead polymer chains. The initial polymerization volume of the simulation was 105 nm3. The ratio of monomer to initiator concentration and initiator to polymer particles were 500 and 2.5, respectively. As the conversion was increased from 20% to 75%, the molecular weight distribution became bimodal. The maximum of the second peak of the bimodal distribution moved to higher molecular weights as the conversion was increased. Tetra-functional branching had little effect on number average molecular Weight, but it had a significant effect on weight average molecular weight. Therefore, weight average molecular weight is more suitable for characterization of emulsion polymerization systems with high extent of tetra-functional branching.