This paper presents more convenient deflection function that properly substitutes slow converging series function by Naiver for simply supported quadrilateral thin plate. The formulation of the deflection function was conducted by using variational approach. During formulation, Rayleigh-Ritz procedures was used and coordinate function that satisfies kinematic boundary condition and the minimum potential energy principle was developed. In addition to coordinate function, modification factor was applied to the main function for curve fitting purpose. To validate the current study, comparisons were done with Navier's solution and finite element analysis. The results shown that the deviation for mid-point deflection of current study was very small, i.e. around 0.03%, from Navier's solution. Furthermore, it has been investigated that finite element analysis also resulted closer result with current study.