A large number of studies have been conducted to investigate macroscopic responses of gravelly soils using large-scale triaxial compression tests; however, particle-level responses of gravelly soils under triaxial compression remain poorly explored. Here, we reported a series of large-scale triaxial compression tests on uniformly graded gravels subjected to a low to medium range of confining pressures (200–800 kPa). We further investigated the particle-level responses of the gravels using the combined triaxial compression test with computed tomography (CT) scan. A computer image analysis program was developed to analyze the CT images, providing visualized and quantitative particle displacement and rotation. Based on the experimental results, the stress-strain relationships at different confining pressures and the microscopic characteristics of the gravels (e.g., changes in internal structures) under different deviator stress levels were discussed. The changes in particle-level responses were linked to those in macroscopic responses. From both macroscopic and microscopic analyses, some insights into the underlying mechanism of the stress-strain relationships were provided.