AbstractThis paper presents a series of single fiber pull-out tests on steel and glass fiber embedded in a cementitious matrix. First, the phase of fiber sliding in the concrete matrix was defined using a shear lag model; then, governing equations for the sliding mechanism were derived using the analytical model. FEM analysis was then used to calibrate the frictional coefficient. Further analysis related to the pressure exerted on the fiber by the surrounding concrete revealed that a smaller fiber diameter would improve bond strength. In terms of the relationship between pressure from the surrounding concrete and the diameter of the fiber, the results showed that a smaller fiber diameter would increase the matrix pressure; improved frictional bond strength and behavior would be expected compared to larger fiber diameters. The results also showed that decreasing fiber diameter by 75% would improve the frictional bond strength by as much as 300% for smooth and straight steel fiber, whereas reducing the fiber diameter by 66% would improve the frictional bond strength by 190% for single glass fibers. In addition, the results showed that a smaller diameter of the fiber improves interfacial properties between the fiber and concrete in fiber-reinforced concrete (FRC) composites.