In this paper, microstructure of concrete is investigated using metakaolin (MK) as cement replacing material and Polyvinyl Alcohol (PVA) fibers. Total ten (10) mixes of concrete are examined by varying PVA fiber aspect ratio. It was found that MK refines the pore structure, improves interfacial transition zone (ITZ) due to its pozzolanic effects, reduces portlandite (Ca(OH) 2 ) content and bridges the gap between matrix and aggregates due to finer particle size. Due to improvement in ITZ, the compressive strength was improved. There was no indication of Ca(OH) 2 around the PVA fibers in the presence of MK and the interface between the fiber and matrix was observed very narrow. © 2015 AIP Publishing LLC.