Alkyl esters of long chain fatty acid are called biodiesel. These esters can be obtained from the transesterification of triglycerides with methanol/ethanol. This work investigated the possibility of using KF/gamma-Al2O3 as heterogeneous catalysts for the transesterification of cottonseed oil with methanol. The operation variables used were methanol/oil molar ratio (6:1-18:1),catalyst concentration (1-5 wt %), temperature (50-68 degrees C), and catalyst type. The biodiesel with the best properties was obtained using a methanol/oil molar ratio of 12: 1, catalyst (4%), and 65 degrees C temperature with the catalyst KF/gamma-Al2O3. The strongest basic sites (superbasic) promote the transesterification reaction also at very low temperature (65 degrees C), while the basic sites of medium strength require higher temperatures to promote the same reaction.