Artificial three-dimensional (3D) video technology has generated significant interest in recent years. However, there are concerns about side effects such as eye strain that may result from viewing 3D video images. This visual fatigue is assumed to be related to a mismatch between convergence distance and focal length that occurs while viewing 3D content. In this study, we investigated the relationship between this mismatch and visual fatigue. Two subject groups completed an easy task while viewing a 3D image containing a convergence distance and focal length mismatch, and while viewing an image without such a mismatch. After the task, visual fatigue was evaluated using a flicker test and a simulation sickness questionnaire (SSQ). Experimental results demonstrated that viewing a 3D video image with the mismatch resulted in more visual fatigue than doing so without the mismatch.