AbstractMetakaolin (MK)-based geopolymer is an emerging alterative to cement, but the quantitative understanding of its pore structure is lacking. Prior studies are limited to phenomenological investigations of the influence of activator modulus and concentration, liquid/solid ratio, or curing temperature. This study characterized the pore structure of a MK-based geopolymer using mercury intrusion porosimetry, gas adsorption, and scanning electron microscopy and revealed the quantitative correlation between its pore structure and geopolymerization degree. Most pores in the MK-based geopolymers were from 10 to 100 nm—noticeably smaller than that of OPC paste (<1,000 nm). A denser microstructure can be achieved by increasing the degree of geopolymerization through lower activator modulus or higher activator concentration. The liquid/solid ratio and curing temperature also influenced the geopolymer microstructure mainly by changing the initial distance between MK particles and affecting the water loss during curing, respectively. These findings provide guidance for studying the shrinkage and chloride resistance of geopolymer concrete, among other properties.