Cross-frequency coupling (CFC) between the theta (4-12Hz) phase and the amplitude of gamma (30-100Hz) oscillations occurs frequently in brain. However, the function of theta-gamma coupling in rat medial prefrontal cortex (mPFC) in working memory remains unclear. To address this issue, we studied 16-channel CFC in local field potentials (LFPs) recorded from the mPFC of over-trained rats as they performed a Y-maze working memory task. During this course, the amplitude of the low gamma subband (30-60Hz) was more strongly modulated by theta phase during the correct trials than in the error ones. Meanwhile, more channels showed higher theta-low gamma coupling levels during the correct performance. By contrast, the strength of theta-high gamma coupling did not significantly increase during the working memory task in both correct and error trials, indicating an insignificant correlation with the performance validity. These findings suggest a role of mPFC theta-low gamma coupling in working memory.