An ultra-compact hybrid-integration receiver optical subassembly (ROSA) with four channels is demonstrated in our laboratory with the size of 23.3 mm × 6.0 mm × 6.5 mm. The ROSA is comprised of a planar lightwave circuit (PLC) arrayed waveguide grating (AWG) chip, a top-illuminated positive-intrinsic-negative photodetector array chip, and a three-dimensional microwave circuit that is specially designed for compact packaging. For each transmission lane, the 3 dB bandwidth of the ROSA is up to 20 GHz, and the maximum responsivity is up to 0.53 A/W. The proposed package structure can be used for smaller package sizes and would be an easy assembling solution for 100 GbE optical communication devices.