In October 2002, he auditioned to become a singer through the Starlight Casting System, and was initially placed in a four-member group called "Four Seasons" along with Jaejoong, Yunho and Kangin.
While Jaejoong and Yunho went on to debut as TVXQ in December 2003, Heechul and Kangin joined ten other trainees under S. Entertainment to form the group "Super Junior 05", the first generation of Super Junior.
I don’t know how to do that But if you’re ever sick, don’t call 911 but call me Tell me to cry, I’ll cry, tell me to smile, I’ll smile, tell me to roll around, I’ll roll around I’m getting nervous, I’m getting nervous Who are you? Hold me tight before I kiss you Before my heart lets you go Say what you want Say what you want What is it that you really want?
In 2011, he formed a project group with label mate Jungmo of TRAX called M&D (meaning "Midnight and Dawn" or "Miari and Dangae-dong", after the members' hometowns).