Slip-resistant bolted connections are often used in metal structures. Their application is of particular interest in aluminum structures when there is a need to avoid heat-affected zone (HAZ). The behavior of slip-resistant connections is highly dependent on the coefficient of friction between surfaces and the preloading force in bolts. The paper presents the investigation of the behavior of the joint made of aluminum AW-6060-T66, which is available on the market in Bosnia and Herzegovina and connected with high-strength M 20 bolts, and the determination of the coefficient of friction for three different treatments of friction surfaces, namely elements with untreated surfaces, elements with surfaces treated with a steel brush and elements with plastic-coated surfaces. The sliding forces of three samples for each surface treatment were experimentally tested and compared to the theoretical values according to Eurocode 9. Based on the test results, characteristic values of slip factor were statistically determined for each of the three different treatments of friction surfaces.