This study aimed to reveal the attitudes of the students who were subjected to measurement and evaluation in an online testing environment towards the computer based testing (CBT) platform and the factors that affect their attitudes. It also examined the students' cognitive loads in the exam designed with multimedia elements from message design principles by using the facilities of this platform. The study was conducted with a case study design. The participants consisted of 32 students. Participants took a previously used and redesigned English achievement test based on the multimedia design principle. Then, cognitive load scale and attitude towards computer scale were implemented. In addition, seven volunteer participants were interviewed. The results of the study showed that factors such as students' equipment advantage, time, success, flexibility, individuality and less mental effort affected the participants' attitudes towards the CBT environment. A significant difference was found between the scores of the participants in the cognitive load scale and the duration of the exam. There was a low and positive correlation between the cognitive load score of the participants for the listening test and the time to complete these questions, and a low and negative correlation between the correct answer scores of the participants in the listening test.