The ability of human memory declines due to normal aging and cognitive diseases, which means that everyone will eventually be affected by this problem. Fortunately, memory ability can be improved by training, and early detection and treatment can even actively prevent serious memory loss. Based on this principle, we aimed to identify a method for estimating the memory ability in order to slow the progression of memory loss. Numerous studies have found that coupling between the theta and gamma bands (also referred to as theta-gamma synchronization) reflect memory processes, with this being more pronounced when maintaining working memory. This study measured the extent of theta-gamma synchronization, and used a wavelet transform to observe the activity in the theta and gamma bands during a visuospatial memory task. The findings showed that there was a pronounced change during the encoding and retrieval phases of a working-memory task, and a significant correlation between the rate of correct responses and the synchronization index in the parietal brain area. We propose that the extent of theta-gamma synchronization can be used to estimate the working-memory ability. The further application of theta-gamma synchronization in both clinical and home situations may be expected in the future.