A program method of a nonvolatile memory device includes loading first word line data to be stored in first memory cells connected to a first word line and second word line data to be stored in second memory cells connected to a second word line; setting up upper bit lines according to the first word line data; turning off bit line sharing transistors after the upper bit lines are set up; setting up lower bit lines according to the second word line data; performing a first program operation on the first memory cells using the upper bit lines; turning on the bit line sharing transistors; and performing a second program operation on the second memory cells using the lower bit lines. The bit line sharing transistors electrically connect the upper bit lines and the lower bit lines in response to a bit line sharing signal.