Abstract: Laboratory observations of five years long have been made on the micro-pore structures of MgO concrete admixed with and without fly ash respectively in this study, using mercury intrusion porosimetry to prompt application of the concrete with magnesium oxide or MgO concrete. Concrete without MgO admixture was used as a reference. The results showed that the micro-pore structure of concrete was improved continuously with age when mixing with appropriate MgO content while it was further improved at long age when mixing with appropriate MgO and fly ash contents.