In order to increase the usage of semi-soft coking coal, various technologies have been developed and commercialized. Among these, an additive for improving coal caking property (hereinafter referred to as a binder) is promising. Here the mechanism of binders for improving coal caking property and the influence of binder addition to coal with different particle size on caking property and coke strength were investigated. The thermogravimetric analysis add dilatometry tests have suggested that as for coal-derived binder, the gas generating from the binder interacts with coal before coal starts to soften and that coal is reformed in-situ during heating by the interaction, which leads to enhancement of caking property. On the other hand, as for petroleum-derived binder, the experiments have implied that the reason why the petroleum-derived binder enhances coal caking properties is that the gas generating as a result of pyrolysis of the petroleum-derived binder "in" the plastic coal helps the plastic coal to swell and become more fluid. Moreover, dilatation differs greatly with coal types and particle size fractions. When the specific dilatation volume of fine coal is equal to that of coarse coal, the coke structure becomes more homogeneous, which leads to high strength coke structure. There is a possibility that under the binder addition ratio being constant, coke with maximum DI can be obtained by adding binders to fine coal and coarse coal separately so that the specific dilatation volume of fine coal becomes equal to that of coarse coal. (C) 2017 Elsevier B.V. All rights reserved.