Curve diffuser is frequently used in applications such as HVAC, wind-tunnel, gas turbine cycle, aircraft engine etc. as an adapter to join the conduits of different cross-sectional areas or an ejector to decelerate the flow and raise the static pressure before discharging to the atmosphere. The performance of the curve diffuser is greatly affected by the abrupt expansion and inflection introduced, particularly when a sharp 90o curve diffuser is configured with a high area ratio (AR). Therefore, the paper aims to numerically investigate the effect of the expansion direction of AR=1.2 to 4.0 curve diffuser on loss characteristic and flow rectification. 90o curve diffuser operated at inflow Reynolds Number, Rein=5.934 � 104 to 1.783 � 105 was considered. Results show that pressure recovery improves when the area ratio increases from 1.2 to 2.16 for both 2D expansion (z-direction) and 3D expansion (x-and z-direction). On the other hand, the increase of inflow Reynolds number causes the flow uniformity to drop regardless of the expansion directions. 3D expansion (x-and z-direction) curve diffuser with AR=2.16, operated at Rein=8.163 � 104, is opted as the most optimum, producing the best pressure recovery up to 0.380. Meanwhile, 2D expansion (z-direction) curve diffuser of AR=2.16,, operated at Rein= 5.934 � 104, is chosen to provide the best flow uniformity of 2.330 m/s. 2D expansion (x-direction) should be as best avoided as it provides the worst overall performance of 90o curve diffuser. © 2021, Penerbit Akademia Baru. All rights reserved.