In this study, the physicochemical characteristics and fuel performance of castor-linseed biofuel were studied in different mixing ratio by petroleum fuel. Firstly, the properties of the mixed castor oil biofuel-linseed oil biofuel were investigated including: density, specific gravity, viscosity, boiling point, pour point, cloud point, flash point, fire and smoke points. The change in the studied properties by gradual addition of the prepared biofuel to petroleum fuel was investigated and correlated to several factors. The study showed the ability of mixing castor-linseed mixed biofuel and petroleum fuel up to 30% (biofuel) with the stability of the required properties of the final fuel to be in accordance with the ASTM fuel specifications.