In this research, the negative perceptions of undergraduate students regarding modern physics course and the causes of their negative perceptions have been investigated. For this investigation, a qualitative and quantitative method (mix method) was chosen for data collection and analysis. The study group of the research consists of a total of 169 (21-25 years old) undergraduate students studying at a state university in Turkey. Focus group interview, repertory grid technique and individual interview were used for gathering data. According to results, seven negative perceptions were determined regarding the modern physics course and four main themes were specified the causes of negative perceptions of students related to modern physics.