This study aims to investigate the gender differences of college students concerning the levels of their understandings on the six major concepts in Quantum Mechanics (QM) and the degree of certainty of their knowledge. For this purpose, 182 physics major college students responded to the revised Quantum Mechanics Conceptual Survey (QMCS) and the Scale for Degree of Certainty on their Knowledge (SDCK). In the results, male students presented statistically-significant higher mean scores on both the QMCS and the SDCK. Specifically, female students held insufficient knowledge of 'wave-particle duality' and the 'wave function'. In addition, a significant correlation between the scores on the QMCS and the SDCK was observed in both gender groups. The students presented decent levels of understanding on the concept of 'quantization of states' with a high level of certainty whereas they struggled to understand the concepts of 'wave-particle duality', 'tunneling effect' and 'Schrödinger equation' with a low degree of certainty.