Numerical analysis and air-water test were performed in order to evaluate gas-liquid separation characteristics in a BWR steam separator. Centrifugal force and flow quality were simulated in the test. Simulation of plant operation conditions was not sufficient at low volumetric flux due to large effect of gravity force. Pressure loss coefficient agreed with the plant conditions within 10%. Carryover agreed well with plant conditions for a current separator but was under-estimated for a separator with small vane angle. Calculated gas-liquid behavior agreed well with plant condition data in the 1^st stage but calculated results over-estimated carryover. Combination of calculated results in the 1^st stage and simulation test results in the 2^nd and 3^rd stages gave good prediction for carryover.