Palm cooking oil consumption in Indonesia is very high, and as a result, the used cooking oil which mostly ends up as the waste is also high. Direct discharge of waste cooking oil (WCO) into the environment causes serious environmental pollution problems. WCO contains triglyceride and free fatty acid, which can be converted into biodiesel. In this study, the conversion of WCO into biodiesel was conducted using non-catalytic subcritical methanol process. The effect of the ratio of WCO to methanol (w/v), temperature, and pressure on the recovery of biodiesel was investigated under constant reaction time of 4 h. Based on the Response Surface Methodology (RSM); temperature, pressure, and WCO to methanol ratio (w/v) gave a significant effect on the recovery of fatty acid methyl ester (FAME). From the experimental result, the maximum FAME recovery obtained was 93.29% with the purity up to 97% (200 °C, 5.5 MPa, 3:10), while the predicted recovery calculated by RSM was 91.93% with the optimum condition: 200 °C, 5.5 MPa for WCO to methanol ratio 0.3061 (w/v). The experimental verification showed satisfactory agreement between the observed and predicted values with only 1.36% of error. Therefore, subcritical methanol has good prospects to be applied further in lieu of conventional process to utilize waste oil with high free fatty acid content.