Effects of increased injection pressure and advanced injection timing on the performance, combustion characteristics, and exhaust emission properties of a direct injection, naturally aspirated diesel engine operating on methyl soyoil ester (IV = 125 to 135) and methyl tallow ester (IV = 47 to 53) were investigated. A 3-way factorial (2 x 2 x 3) experimental design was employed in this engine performance test. The whole experiment consisted of two levels of injection pressure (18.6 MPa and 24.1 MPa), two levels of injection timing (19 deg. BTDC and 14 deg. BTDC), and three different test fuels (Phillips No. 2 diesel, methyl soyoil ester and methyl tallow ester), The Phillips No. 2 diesel fuel was used as the reference (baseline) fuel in this test. The physical and chemical properties of the test fuels were determined in accordance to the standards established by the ASTN and AOCS;Results of this investigation indicated lower exhaust emissions, despite a slight increase in the brake specific fuel consumption, with the engine operating on the ester fuels as compared to No. 2 diesel fuel at the manufacturer's injection pressure-timing setting (18.6 MPa-14 deg. BTDC). The overall performance and exhaust emission properties of the engine operating on the ester fuels at advanced injection timing were improved and better than when operating at increased injection pressure. Poor fuel combustion near the maximum operating power level was observed with the engine operating on ester fuels at increased injection pressure. Increased injection pressure resulted in a spray of smaller cone angle but greater penetration and better atomization. The occurrence of complete combustion was suppressed at the condition of high fuel flow due to the lack of oxygen within the spray envelope. Fuels in some locations within the spray envelope that were too rich to burn escaped as unburned hydrocarbons, or burned incompletely causing high carbon monoxide and smoke levels in the exhaust emissions.