The classical tropical cyclone (TC) maximum intensity theory of Emanuel suggests that the maximum azimuthal wind of TC depends linearly on the ratio of surface exchange coefficients for enthalpy and momentum (Ck and Cd). In this study, a series of sensitivity experiments are conducted with the three-dimensional Cloud Model 1 (CM1), by fix- ing the ratio of Ck/Cd but varying the specific values of Ck and Cd simultaneously. The results show significant vari- ations in the simulated TC maximum intensity by varying Ck and Cd, even if their ratio is fixed. Overall, the maxi- mum intensity increases steadily with increasing Ck and Cd when their value is smaller than 1.00 × 10?3, and then this increasing trend slows down with further increases in the coefficients. Two previous theoretical frameworks—one based on gradient wind balance and the other incorporating the unbalanced terms—are applied to calculate the max- imum potential intensity (PI). The calculated value of the former shows little variation when varying the specific val- ues of Ck and Cd, while the latter shows larger values with increases in both Ck and Cd. Further examination suggests that the unbalanced effect plays a key role in contributing to the increasing intensity with increasing Ck and Cd.