We experimentally realize a nanoscale all-optical diode in a photonic crystal heterostructure with broken spatial inversion symmetry, performing independent of optical nonlinearity. The physical mechanism lies in unique dispersion relations of the photonic crystal and the transition of incident light between different electromagnetic Bloch modes. An ultrahigh transmission contrast of ${10}^3$ order, a large operating bandwidth of over $50\mathrm{nm}$ , and an ultralow photon intensity of less than $10\mathrm{kW}/{\mathrm{cm}}^2$ are reached simultaneously.