The effects of palm oil fuel ash (POFA) as micro-filler on interfacial porosity and pore size distribution of polymer concrete (PC) is the main aim of this chapter. Beginning with a brief introduction about the topic, the materials and method used in this study are explained. Two categories of fillers were involved in this study, fine-micro filler (ground POFA and is paired with calcium carbonate), and coarse micro-filler (unground POFA and is paired with silica sand). It is revealed that the replacement of overall types of micro-filler at different filler content decreased the average pore diameter of PC significantly, except for PC incorporating unground POFA. Additionally, incorporation of fine-micro filler with dispersion characteristic could significantly reduce the interfacial porosity of PC as compared to incorporation of coarse micro-filler in PC.