Provided is a user authentication method using a natural gesture input. The user authentication method includes recognizing a plurality of natural gesture inputs from image data of a user, determining number of the plurality of natural gesture inputs as total number of authentication steps, determining a reference ratio representing a ratio of number of authentication steps requiring authentication pass to the total number of the authentication steps, determining an actual ratio representing a ratio of number of authentication steps, where authentication has actually passed, to the total number of the authentication steps, and performing authentication on the user, based on a result obtained by comparing the actual ratio and the reference ratio.