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The vertical cavity surface emitting laser (VCSEL) is a very unique semiconductor laser diode since it operates in low current and can be tested during the fabrication process. Therefore, VCSEL is more preferable than the conventional edge-emitting laser diode for certain applications. However, the cavity design of a single-transversal-mode VCSEL is not straight forward. This research will explore the single-transversal-mode VCSEL design by using an Effective Index Method (EIM). With EIM, the structural dimension of VCSEL will be reduced from three dimensions into two dimensions. EIM used in this paper is a combination of one-dimensional Transfer Matrix Method (TMM) and two-dimensional Finite Element Method (FEM). By using the TMM and FEM mode solvers blended in EIM, we can find the best cavity design which will produce a single-transversal-mode VCSEL with Multi Quantum Well (MQW) active medium. The VCSEL being studied has cladding design similar to photonic crystal which consists of well arranged holes in the size of sub-micrometer to micrometer. Simulations performed in this study reveal that single-mode VCSEL can be attained by adjusting the cavity parameters such as core radius and the distance between holes in the cladding. The simulations show that core radius is the most influential parameter for the mode characteristic. For a typical VCSEL structure based on GaAs materials under study, core radius of 0.25 μm to 1.2 μm can create a single mode VCSEL. Pitch size is also influential for the mode characteristic. The simulations also reveal that for single mode VCSEL, the maximum pitch size is 7 μm for core radius from 0.25 to 1.2 μm.