Screening and catalytic activity of alkaline modified zirconia i.e. Mg/ZrO2, Ca/ZrO2, Sr/ZrO2. and Ba/ZrO2 as heterogeneous catalyst in biodiesel production from waste cooking oil (WCO) have been investigated. The catalysts were prepared via wet impregnation of alkaline nitrate salts supported on zirconia. Physico-chemical characteristics of the catalysts were analyzed by BET surface area, XRD, FESEM and CO2-NH3-TPD. Among the catalysts screened, Sr/ZrO2 exhibited higher catalytic activities. Characterization results disclosed Sr/ZrO2 catalyst possessed balanced basic and acid site concentrations with its pore volume, surface area as well as pore diameters suitable for biodiesel production. The balanced active sites facilitated simultaneous transesterification and esterification of WCO. A plausible mechanism has been suggested for the simultaneous reactions. The effects of operating process conditions such as methanol to oil molar ratio, reaction temperature and catalyst loading on biodiesel production in the presence of Sr/ZrO2 were investigated. Methyl ester (ME) yield at 79.7% was produced over 2.7 wt.% catalyst loading (Sr/ZrO2), 29:1 methanol to oil molar ratio, 169 min of reaction time and 115.5 degrees C temperature. (C) 2011 Elsevier B.V. All rights reserved.