A method of user-defined key setting in a portable electronic communication device. Corresponding relationships between keys and functions is recorded in a first mapping table and the portable electronic communication device executes the functions accordingly. When a user-defined key is to be set, a first key input and a function input are first received. The corresponding relationship between the first key input and the function input is then recorded in a second mapping table. The portable electronic communication device executes the functions according to the second mapping table.