The mechanical behavior of granular materials is highly dependent on the arrangement of particles, particle groups, and associated pore spaces. Changes in the internal structure due to large deformation may cause changes in the mechanical behavior. The changes include: particle sliding, rolling, and interaction; shear band formation; and fabric anisotropy. During those changes, stick-slip behavior may take place between the granular particles. The objective of the thesis is to study the factors that influence the stick-slip behavior of granular materials. The influence of particle size, uniformity, confining pressure, density, and strain rate are investigated in this thesis. A series of axisymmetric triaxial tests were performed on glass beads to study the shear strength of granular materials. Sizes that were used are labeled as Very Small (d = 0.15 – 0.25 mm), Small (d = 0.75 – 1.00 mm), Medium (d = 1.55 – 1.85 mm), Large (d = 3.30 – 3.60 mm), and Well-graded (d = 0.09 – 1.55 mm). The confining pressures were 25, 100, 250, and 400 kPa. The load oscillations that appeared in the stress-strain results were analyzed to find the causes of the stick-slip behavior. To study the internal structure of the particles, two axisymmetric triaxial tests were performed on the glass beads under low confining pressure (25 kPa). The specimens were composed of Very Small, Medium, and Well-graded. The specimens were scanned before and after compression using a X-ray computed tomography system. In general, a slight post peak principle stress softening was observed as well as a continuous volume increase (dilation) even at relatively high strains. This appears to be caused by the uniform shape of the spherical particles. The load oscillations that appeared in the very small, small, and well-graded beads are due to the stick-slip phenomenon. From the computed tomography analysis, the specimens showed a bulging deformation mode. This is because the particles roll each other continuously during compression; they do not interlock. In the medium beads after compression, columns of beads were found in the specimen to support the theory of the stick-slip behavior.