Waste cooking oil (WCO) can be used as an energy source to reduce fossil fuel consumption; however, WCO has high viscosity which restricts direct use in diesel engines. Emulsification techniques can be employed for fuel preparation by selecting suitable surfactants and WCO, diesel and butanol or ethanol ratios using a pseudoternary phase diagram. Results indicated that a WCO-diesel fuel blend (WDF) with butanol required no surfactants for homogeneous phase blending compared to WDF with ethanol. Phase behavior of WDF-ethanol blends with hydrophobic surfactants was more suitable than WDF-ethanol blends with more hydrophilic surfactants. Kinematic viscosity of WDF blends depended on the amount of surfactant and its hydrophile-lipophile balance (HLB) value. WCO content was determined as a significant factor contributing to WDF viscosity (p < 0.05). Biofuel blends containing 70% v/v diesel fuel, 15-20% v/v WCO, and 15-20% v/v ethanol with Dehydol LS1 or Span 80 at ethanol:surfactant ratio of 9:1 were selected for further analysis of relevant fuel properties. All selected WDFs gave emulsion droplet size at 200 nm or less. Other properties such as water content, carbon residue and copper strip corrosion met values specified by diesel and biodiesel fuel standards. Acid value of WDFs exceeded the standard due to high acid content of WCO. WDFs with ethanol/surfactants showed higher oxidative stabilities than WDFs with butanol. In terms of safety, conditions for storage and transport should be applied for WDFs because of their low flash point. WDFs obtained by this technique may be used in unmodified diesel engines but engine performance should be monitored.