By using a distorted-wave Born exchange approximation, the direct ionizarion cross section of boron-like ions by electron impact have been calculated. The interferences between direct and exchange amplitudes make important contribution in the process of ionization. The calculated results indicate that the "Natural Phase" approximation is better than the "Maximum Interference" approximation, while the latter agrees better with experiments for some other species of ions. Compared with Coulomb-Born approximation of Moores et al., our calculation gives better agreement with the experiment data available.