We propose and demonstrate a direct integration of a wavelength-scale III-V nanolaser onto a silicon-on-insulator (SOI) waveguide. By employing high-precision microtransfer printing techniques, with an optimally designed photonic crystal nanolaser structure, we experimentally achieved a coupling efficiency of 83% between the InGaAsP nanobeam laser and the SOI waveguide. Our III-V nanobeam laser is designed as an asymmetric one-dimensional photonic crystal cavity, which allows unidirectional coupling to the combined III-V nanobeam waveguide with high efficiency. Through the compact vertical coupler in the region where the III-V and SOI waveguides overlap at the optimal length of 3.2 mu m, 88% of the light from the printed III-V nanolaser can theoretically be coupled to a vertically integrated SOI waveguide.