This study aims to investigate the effects of different design tasks on the cognitive load level of instructional designers during the process of designing a learning activity in a 3D multi-user virtual environment (MUVE). The sample consisted of 16 undergraduate students who were experienced in the areas of instructional design, computer programming and graphic design. The designers were assigned to the design teams with 5 or 6 members and conducted a collaborative instructional design project to develop a solution to an ill-structured problem on the OpenSimulator platform for 8 weeks. Following the implementation, the designers were administered the Mental Effort Rating Scale regarding the 11 design tasks in the 3D MUVE. The results showed that the designers were cognitively overloaded while performing the design tasks in this environment. When the cognitive load scores were examined, it was found that the preparation of the learning scenario, uploading user-created 3D models, content creation, modelling of the draft view of the designed environment and 3D model design tasks were found as the most challenging tasks in terms of cognitive effort. On the other hand, it was found that the tasks of making animation, creating non-player characters and designing an interactive menu led to relatively less cognitive load.