Low plasticity, high bearing capacity, low settlement, etc. are the preferred properties for most engineering projects. Alluvial soils are problematic soils because of low bearing capacity, high organic matter content, and high void ratio so they do not meet the preferred condition for engineering projects. It has been necessary to improve unsuitable materials to make them acceptable for construction. Fly ash (FA) has earlier been used for stabilizing roads due to its high content of calcium and silicate oxides which give puzzolanic properties and thus high compression strength. In this research, fundamental engineering properties, compaction behaviors of three types of (fine, medium, and coarse) alluvial deposits, and the effect of fly ash on compaction behavior of these alluvial soils are presented. Alluvial soil is taken from Çiğli, Balatçık (Izmir, Turkey). To determine geotechnical index properties; wet sieve analysis, plastic limit, liquid limit, specific gravity, standard compaction tests were conducted. In order to determine the effect of fly ash on compaction behavior of alluvial deposits, three different samples (fine < 0.425mm, medium < 2mm, and coarse < 4.75 mm) are prepared and 10%, 15%, 20% fly ash by dry weight of soil is mixed and standard proctor test is performed. As a result of laboratory tests, the liquid limit, plastic limit, and plasticity index values obtained as 38.3%, 25.7%, and 12.6%, respectively. The specific gravities for fine, medium, and coarse samples are 2.68, 2.67, and 2.66, respectively. According to the results of wet sieve analysis and consistency limit tests, it was stated that the soil contains large amounts of sand and clay. The washed sieve analysis and consistency limit tests results were evaluated according to USCS. The conducted test results have shown that maximum dry unit weight for fine, medium, and coarse soils are 16.9, 19.35, and 19.55 (kN/m3), and optimum moisture content for fine, medium, and coarse samples are 17, 11, 10.5% respectively. Generally, by increasing the content of FA, maximum dry unit weight decreased and optimum moisture content increased for all three types of alluvial soil. By increasing FA to 20%, maximum dry unit weight of medium and coarse soils decreases 1.5% and 2%, respectively.