Abstract:  This paper briefly introduces the Proctor Test principles on granular soils. Factors that are potentially affecting the laboratory compaction test results, such as, gradation, material properties, sample preparation methods and methods of compaction, are evaluated in detail. The effect of coarse gain soil content in the granular soil samples on the Proctor Test results are summarized from test results from five different sources. Combined from the rest results as well as the literature review, the following conclusions can be made: 1) The critical factor that affecting the optimum moisture content is not the size of the coarse grains but the content of the coarse grain soils; 2) linear relationships are existed between the optimum moisture content and coarse grain coarse, the optimum moisture content and fine grain content; 3) the linear relationships are formulized.  
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Key words: coarse grained soils, compactive effort, optimum moisture content, matrix effect, linear regression