Magnetic properties such as coercive force, initial susceptibility, etc., are structure sensitive parameters. Especially, the coercive force depends on the existence of crystal defects such as dislocations and grain boundaries. In this study, the grain-size dependence of coercive force was investigated in an iron film, 150 nm in thickness, deposited by sputtering method on a (001) KCl substrate at room temperature. The grain size of the film was changed from 15 to 120 nm by annealing in a vacuum. The coercive force increased with the increase of grain size and was proportional to the square of grain size. The magnetic domain had a ripple structure when the grain size was smaller than 50 nm. A grain size larger than 50 nm led to an irregular domain wall configuration and the formation of domain walls in the grain. (c) 2005 Springer Science + Business Media, Inc.