A semi-analytical model for predicting heat transfer and pressure drop in annular flow regime for saturated flow boiling in a horizontal microtube at a uniform heat flux has been developed based on a one-dimensional separated flow model. More than 600 two-phase heat transfer, 498 two-phase pressure drop, and 153 void fraction experimental data points for annular flow regime were collected from the literature to validate the present model. The collected data were recorded for various working fluids, R134a, R1234ze, R236fa, R410a, R113, and CO2, for round macro- and microsingle horizontal tubes with an inner diameter range of 0.244mmDh3.1mm, a heated length to diameter ratio of 90Lh/Dh2000, a saturation temperature range of 10Tsat+50C, and liquid to vapor density ratios in the range 6.4f/g188. The model was tested for laminar and turbulent flow boiling conditions corresponding to an equivalent Reynolds number, 1900Reeq48 000, and confinement number, 0.27Cconf3.4. Under the annular flow regime, the present model predicted the collected data of the heat transfer, pressure drop, and void fraction with mean absolute errors (MAE) of 18.14%, 23.02%, and 3.22%, respectively.