Abstract: Three-dimensional ensemble Monte Carlo model has been accomplished using C programming language to study the impact of different cathode aperture sizes to micro-hollow cathode discharges in argon under constant pressure and voltage. After adding an external magnetic field in a micro-hollow cathode discharge, the distribution of electron density has little change in the opposite-cathode direction and a more narrow drift range in the axial and the lateral direction, and electron energy becomes lower, but the applied magnetic field has a less obvious effect on high-pressure micro-hollow cathode discharges than on low-pressure discharges on the whole. The results of this work have some guides on the mechanism of the microhollow cathode discharge plasma.