In this article, the role of nano-size calcium carbonate in penetration resistance of medium- density polyethylene (PE) was investigated through experiments. In order to study the penetration resistance of PE and its nanocomposites, perforation test was carried out. The results of tests showed that penetration resistance depends strongly on calcium carbonate amount. As a matter of fact, addition of CaCO3 to PE increases resistance against penetration as CaCO3 amount reaches to 5 percent of weight. Stereomicroscope was used to evaluate the damage and plastic zone around the perforated area in all the samples including neat polyethylene and its nanocomposites. The plastic zone was measured using an image analysis as an effective technique. The results of image analysis techniques proved that the addition of calcium carbonate to PE makes a damaged zone around the perforated area. The results of microscopic evaluations showed that the area of plastic zone rises as the amount of calcium carbonate increases up to 7.5 percent of weight. By increasing the amount of CaCO3, resistance against penetration decreases more and some micro cracks appear around the perforated area. For further clarification of the fracture mechanism of MDPE nanocomposites, scanning electron microscopy was employed. Fracture surface images showed that when calcium carbonate is higher than 5 percent of weight, agglomeration of nanoparticles occurs, resulting in lower resistance against penetration to the samples.