Recently, the use of crushed sand obtained from limestone-quarried is growing especially in the countries where river sand is not quite available. In the paper, the results from an experimental investigation are presented, which evaluates the pore structure of mortar mixtures where fine aggregate is replaced by Crushed Limestone Sand (CLS). The behavior of pore structure of CLS mortar was compared to that of mortars prepared from Nature River Sand (NRS). A Mercury Porosimeter Intrusion (MIP) is adopted to study the behavior of pore size distribution. Both CLS and NRS mortar with four water-cement ratios and five sand-cement ratios are used to determine pore structure. An analytical model was established to compare changes in pore structure for different water cement ratio and volume of sand, and the model was based on the test results.