This paper reports the experimental observation and numerical simulation of microcapillary formation, which was manipulated by the injection air pressure within microcapillary films (MCFs). Blown film technique was applied to produce low-voidage and high-voidage MCFs. It was demonstrated to be an effective and easy method to control both the capillary size and voidage of MCFs through adjusting injection air pressure. The relationships between the microcapillary size and voidage of 28-capillary films and the injection air pressure were investigated. Empirical equations for predicting the capillary diameter and voidage have been established. Numerical simulations of processing of both single- and 28-capillary films using Polyflow(A (R)) were carried out, and the influence of injection air pressure on the microcapillary formation was confirmed. The simulation results provided an insight of capillary formation and the mechanism of this process was discussed.