The use of the vegetable oils is preferred compared to petroleum-based oil because they are biodegradable and non-toxic. The purpose of this study is to investigate the friction properties of pure soybean oil and 5% boric acid-added soybean oil under boundary lubrication conditions. Tests were carried out under 100 and 300 rpm sliding velocity and 100, 200 and 300 N contact forces at room temperature. Reciprocating test rig was used for the tests. The sliding distance was chosen as 5000 m. The variations of friction coefficient, and weight loss at different test conditions were presented. It was observed that boric acid-added oil reduces the friction coefficient values. The results show that friction coefficient and weight loss values increase with the increasing of load while they decrease with increasing sliding speed.