The correlation between composition, microstructure, and magnetic properties of as-deposited CoFeSiB thin films has been studied by varying the sputtering power density of Co target. The CoFeSiB alloy film shows significantly different microstructure and magnetic properties depending on the Co concentration. When the Co concentration is below 72.2 at.%, the deposited thin films consist of a single amorphous phase and behave magnetically soft. Meanwhile, above 72.2 at.%, the structure consists of Co nanocrystals embedded within the amorphous matrix, and the codeposited samples show the typical antiferromagnetic exchange coupling at the interface between nanocrystals and amorphous matrix.