A program method of a nonvolatile memory device that performs a plurality of program loops is provided. At least one of the plurality of program loops includes dividing a channel of a selected cell string into a first side channel and a second side channel during a first interval and a second interval, turning off a string selection transistor of the selected cell string by applying a string select line voltage of a first level during the first interval, and boosting a first voltage of the first side channel and a second voltage of the second side channel, and turning on the string selection transistor by applying the string select line voltage of a second level different from the first level during the second interval, and performing a hot carrier injection (HCI) program operation on a selected memory cell corresponding to the first side channel or the second side channel.