A study was conducted to investigate the characteristics of fly ash as cement replacement on mortar mixed with Mangima stone and conventional fine aggregates. Since many studies have established that coal fly ash and Mangima stone can be a good substitute for concrete aggregates, the researchers decided to create a new experiment to show the combination of the two materials on a mortar mixture. Several design mixtures were evaluated to compare the test findings. The quality tests of the Mangima stone with conventional fine aggregates and cement with different proportions of fly ash included the following: specific gravity (SSD) and absorption test and sieve analysis (gradation test). It also presents the results of the investigation carried out to evaluate the compressive strength of 1:3 mortar mixes in which conventional fine aggregates was replaced with 50% Mangima stone fine sand by weight which was further modified by partially replacing cement with six percentage ratios i.e.0%, 20%, 40%, 60%, 80% and 100% of coal fly ash. The compressive strength was determined by three (3) trials at 7, 14 and 28 days of age. Test results revealed that the combined use of Mangima stone and conventional fine aggregates exhibited that fly ash can only be utilized as cement replacement on mortar by 20% which attained the type S mortar.