The effect of forming temperature and feed rate on the flaring of AZ31 magnesium cups was investigated in the warm flow-forming process. Flaring is a common phenomenon in flow-forming process whereby the diameter of the work-piece is observed to increase gradually along the flow-formed path. In this study, forward flow-forming process was performed on a cup-shaped work-piece by using two diametrically opposite rollers (approach angle of 20) at a constant thickness reduction. To study the effects on flaring, experiments were performed at feed rates of 0.2mm/rev, 0.4mm/rev, and 0.8mm/rev and at forming temperatures between 50~300°C. Experimental results indicated that feed rate was the most significant parameter influencing flaring while temperature mainly improve the formability of the material. It is believed that a higher feed rate resulted in an increase in metal flow in the circumferential direction because of higher mean circumferential strain and hence increases flaring. Flaring was also observed to be most severe at forming temperature of 300°C and feed rate of 0.8 mm/rev due to the excessive metal flow in the cirucmferential direction of the test-piece. This study clarifies the relationship between feed rate and the extend of flaring and that forming temperature has little influence on flaring from temperature range of 50°C to 250°C.