A fabricating method of a back-illuminated image sensor includes the following steps. First, a silicon wafer having a first surface and a second surface is provided, wherein a number of trench isolations are formed in the first surface, and at least one image sensing member is formed between the trench isolations. Then, a first chemical mechanical polishing (CMP) process is performed to the second surface using the trench isolations as a polishing stop layer to thin the silicon wafer. Because the polishing rate of the silicon material in the silicon wafer is different with that of the isolation material of the trench isolations in the first CMP process, at least one dishing depression is formed in the second surface of the silicon wafer. Finally, a microlens is formed above the dishing depression, and a surface of the microlens facing the dishing depression is a curved surface.