An ultra-simple polarization rotator is demonstrated on SOI platform with self-aligned process to enhance performance repeatability and manufactural yield. The polarization rotation is essentially achieved by the symmetry breaking of a channel waveguide with a single-sided slab. The two-step lithography enabling this structure is fully compatible with the mainstream process flow of Si photonic integration. A polarization conversion efficiency of 93% is obtained at 1560nm in less than 10μm light propagation length. The merit of flat-band operation (> 100nm) by using asymmetric waveguide for polarization rotation is inherited.