Singapore is a developed tropical city-state where is located in Southeast-Asia. With increasing of environmental awareness, The BCA Green Mark Scheme was launched by Building and Construction Authority (BCA) in 2005, to drive buildings towards more sustainable and environment friendly in Singapore. To be eligible for Green Mark Platinum Award for Residential Building, it is required to employ Computational Fluid Dynamics (CFD) ventilation simulation techniques to identify the most effective building design and layout by demonstrating 70% of selected typical units have a weighted average wind velocity of 0.6 m/s. The performance of natural ventilation is affected by a combination of internal (dimension, spatial arrangement and type of openings) and external factors (site location, building orientation and prevailing wind speed). The most of external factors are difficult to be controlled by the developers, the architects and the engineering consultants. Moreover, the majority of current researches in the Southeast-Asia region are focused on the influence of opening configurations and the spatial arrangement while limited exploration is on the influence of the different window types on the natural ventilation performance within apartment units. Hence, the object of this study was to investigate the impact on the natural ventilation performance by different window types (inclusive of sliding window, casement window and hopper window) through CFD simulation, and to contribute as an industry design practice guide for residential development. Due to the limited time span of this study, it was only based on a typical Green Mark Platinum Award residential building. The results showed the casement-window is able to provide higher area-weighted velocity than the other two types within apartment units. Therefore, it can be concluded as casement window is more recommended for Green Mark Platinum Award residential development.