Accuracy is widely recognized as one of the main dimensions in evaluating task-based oral performance. Indicators of accuracy vary in the literature of the effects of pre-task planning on the oral production. Enlightened by previous studies in pre-task planning and third-person singular "-s," this article intends to find out whether the rate of suppliance of thematic verbal agreement increases as a result of pre-task planning, and whether the rate of suppliance of thematic verbal agreement in third-person singular "-s" increases as a result of pre-task planning. A case study is carried on one subject who makes oral descriptions of a same graph which shows the number of people at London underground station from 6:00 to 22:00 on two separate occasions. The results of this study show that pre-task planning may have significant effect on accuracy of L2 oral output in thematic verbal agreement as a whole, but it may not affect L2 learners in their use of third-person singular "-s."