We propose and experimentally demonstrate an all-silicon passive optical diode with low-power consumption and high nonreciprocal transmission ratios (NTRs) based on cascaded opto-mechanical microring resonators (MRRs). As the oxide substrates of the opto-mechanical MRRs are removed, the nonlinear effects in the free-hanging waveguides could be efficiently activated by low optical powers. The operation principle of the optical diode is based on the asymmetric resonance red-shifts of the two MRRs in the forward and backward transmissions, which could be effectively induced by the nonlinear effects. In the experiment, with injecting an optical power low as 0.96 dBm, a high NTR of 33.6 dB and a relatively broad 20-dB bandwidth of 0.11 nm are achieved. The proposed passive optical diode is competent to process optical signals with dominant advantages of CMOS-compatibility, a compact footprint, low-power consumptions and high NTRs, which has significant applications for on-chip signal processing systems, such as logic gates and optical computing.