Simple rules are developed for obtaining rational bounds for two-phase frictional pressure gradient in minichannels and microchannels. The lower bound is based on the Ali et al. correlation for laminar-laminar flow. This correlation is based on the modification of simplified stratified flow model derived from the theoretical approach of Taitel and Dukler for the case of two-phase flow in a narrow channel. The upper bound is based on Chisholm correlation for laminar-laminar flow. The model is verified using published experimental data of two-phase frictional pressure gradient in circular and non-circular shapes. The published data include different working fluids such as air-water and nitrogen-water mixtures, as well as different channel diameters. The bounds models are also presented in a dimensionless form as two-phase frictional multiplier versus Lockhart-Martinelli parameter for different working fluids such as air-water mixture and nitrogen-water mixture. It is shown that the published data can be well bounded.