Optical rib/ridge waveguides are one of the most versatile and hence widely used optical waveguide structures in integrated photonics for different applications. One of the intrinsic features of such waveguide is that single mode condition can be achieved with certain configurations even the overall cross section area of the waveguide is large. For many years, a set of conventional criteria for the single mode condition has been accepted and used without sufficient validation and justification. To the best of my knowledge,  a systematic analysis and validation of the single mode condition for optical rib/ridge waveguides has never been attempted. The situation is somewhat acute for silicon optical waveguide in which the index contrast between the core and the cladding is large. In this presentation, the single mode conditions for optical rib/ridge waveguides are revisited through an analysis applying the full vector mode-matching method. New criteria for the “true-single-mode” (i.e., only HE₁₁ mode exists) and the “quasi-single-mode” (i.e., both HE₁₁ and EH₁₁ modes are permitted) conditions are derived exactly for silicon-on-insulator rib/ridge waveguides with different rib heights. It is observed that strong true single mode condition can be achieved for a wide range of practical design parameters. On the other hand, the quasi and the true single mode conditions start to converge when the rib height increases and/or the index contrast deceases beyond certain values when the semi-vector or scalar approximation is valid. Comparisons with the existing schemes indicate that the conventional criteria widely used in literature are valid only when the rib height is large and/or the index contrast is small in which the true and the quasi single mode conditions are nearly degenerate.