Google Scholar and academic library discovery systems are both popular resources among academic users for finding scholarly information. By conducting an online survey with 975 users from more than 20 public research universities across the United States, this study comparatively investigates how and why academic users use these two resources. Results show that the ways participants used both resources were similar, and both were perceived as highly "accessible" and "useful." Academic library discovery systems' perceived "comprehensiveness", "subjective norm", "loyalty", and "intended use" were higher than Google Scholar, while Google Scholar's perceived "ease of use", "system quality", and "satisfaction" were higher than that of academic library discovery systems.