Based on the Relativistic Multichannel Theory,we have systematically studied the fine structure of nf Rydberg states of alkali atoms. Our calculated differences between the quantum defects μj+ and μj- are in good agreement with measured values, namely normal for Li and Na while inverted for Rb and Cs. We also predict a normal fine-structure for K and an inverted one for Fr, which still have no experimental data currently. Our theoretical calculation shows that the exchange between excited electron and core electrons is responsible for the inversion, which is a purely relativistic effect, while electron correlation is unimportant.