Ethiopia one of the fastest-growing country in the world so from many sectors of the construction industry is one of them and in the construction industry there are many challenges that are generated during construction activities not identify the aggregate size for concrete have a negative impact on the compressive strength of concrete. So this research have identified the effect of aggregate size on compressive strength of C-25 concrete. This study aims at assessing the effect of aggregate size on compressive strength of C-25 concrete, comparing the result with the standard and to provide solution along with determining workability of concrete made from different sized coarse aggregates. This study has given a vast understanding on the effect of aggregate size on compressive strength of C-25 concrete. Significant to major stakeholders like: consultant, contractor, in order to minimize construction defects, cost, and poor quality of materials during construction which may cause defects and also be a guide for other researches. In order to satisfy the above objectives laboratory tests should have to be conducted. These tests are: Sieve analysis, Slump test, Silt content of sand, Specific gravity and absorption of fine aggregate, Specific gravity and absorption of course aggregate, Moisture content of aggregate and Compressive strength of cubic concrete test. After collecting and analyzing all laboratory results researcher have seen that compressive strength and workability have increased with increasing of aggregate size. The researcher has concluded that Coarse aggregate size is directly proportional to the slump (workability) of fresh concrete with constant water-cement ratio and the Compressive strength of a concrete increases with an increase in coarse aggregate size until it reaches 37.5mm and declines above the 37.5mm. And as per the ES and BS size 37.5mm has a mean of 27.15mpa within 28 days of curing this makes it relatively stronger. Researcher recommend that aggregate size 37.5mm can be used for mass concrete structures and size 19.3mm and 63mm with compressive strength of 23.58mpa and 23.65mpa have average strength and can be used to light buildings and to increase the workability of concrete one should increase maximum aggregate size.