The decarboxylation of brown grease (BG) to green diesel hydrocarbons over a 5 wt % Pd/C catalyst was investigated in semibatch and batch reactors. Catalytic deoxygenation of BG under H-2-Ar occurs primarily via decarboxylation with the liquid products of primarily n-heptadecane and n-pentadecane. A 90% conversion of BG in a semibatch mode was obtained in 7 h. In contrast, in a batch reaction the conversion was roughly 40% in the same reaction time. However, by pretreating the "as received" BG with H-2, the conversion in a batch reactor was increased 1.4-fold; and when the H-2 to BG ratio was increased to 3/1 (mol/mol), the conversion was further improved. A complete conversion of BG into green diesel via decarboxylation is possible over 5% Pd/C catalyst at 300 degrees C and 1.5 MPa. This study demonstrates the feasibility of obtaining valuable green diesel biofuel from waste oil.