A font-size changing and printing method and a host and a printer therefor are provided. The present invention receives a before-change font size in a unit of font size generally used in software applications and an after-change font size in a unit which a user is familiar with, converts the inputted after-change font size to correspond to the unit of the inputted before-change font size, searches for the inputted before-change font size out contained in data for printing, changes the searched font size to the converted after-change font size and prints the data for printing. The present invention receives font sizes in the unit which the user is familiar with, and changes the font size and prints data in the changed font size, thereby enhancing a user's convenience.