In this work, we employ atomic layer deposition (ALD) to form Al2O3 layer as an encapsulant for perovskite solar cells (PSCs). Al2O3 layer deposited at temperature as low as 95 degrees C achieves water vapor transmission rate (WVTR) of 1.84 x 10(-2) g m(-2) d(-1) at 45 degrees C-100%RH when thermal ALD is used. In order to test the moisture barrier capability of Al2O3 layer for PSCs, mesoporous perovskite devices, with spiro-OMeTAD or PTAA as hole transport layer (HTM) encapsulated by 50 nm Al2O3 film, are exposed to 65 degrees C-85% RH for 350 h and their stabilities are monitored. We find that the color of perovskite does not change after 350 h of exposure regardless of the type of HTM used. With regards to Th-ALD encapsulated devices, PTAA based PSCs experienced a smaller power conversion efficiency (PCE) drop than spiro-OMeTAD based PSCs after thermal stress at 65 degrees C. This is due to the presence of pinholes within spiro-OMeTAD layer after thermal stress which are not observed in PTAA. Finally, we successfully achieve excellent durability test results for mesoporous (HC(NH2)(2)PbI3)(0.85)(CH3NH3PbBr3)(0.15)/PTAA devices encapsulated by 50 nm Al2O3 with less than 4% drop in PCE after 7500 h (> 10 months) of exposure to 50%RH under room temperature.