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Alkyl ketene dimer(AKD)emulsion sizing agents have been widely used in alkaline papermaking systems for various paper grades from slack-sized to hard-sized papers. Various kinds of AKD sizing agents have been available in Japan. These sizing agents contain AKD waxes with different chain lengths of linear alkyl chains as raw materials. Recently, we have developed novel sizing agent of liquid-type AKD with branched alkyl chains and placed it on the market in Japan. Selections of AKD sizing agent appropriate to each grade of papers and papermaking conditions are crucial to improve paper properties and machine runnability. Thus, to do this, it is important to understand various kinds of available AKD sizing agents including liquid-type AKD. In this paper, physical properties such as melting point and crystallization temperature of AKDs themselves with various chemical structures and their hydrolysis products are described. The physical properties of their emulsion sizing agents are also described. Then effects of chemical structures, i.e. chain length and topological structure, on paper properties such as sizing degree and friction coefficient and on the lamination coating in the finishing process are discussed. Finally combination usages of AKD sizing agent and other paper chemicals such as cationic polymers are exemplified.