Began teaching at a local school after receiving 2nd degree black belt and he also taught at several U.S. Army camps in S. Korea. Joined the ROK Army TKD team at 27 where he was the captain all 3 years he was on the team and lead the team to 3 National President Cup of South Korea titles. Was also training/teaching the army officers at the National Army Headquarters in South Korea during this time. Also trained the instructors that would be going to Vietnam and instruct the U.S. and South Korean soldiers. Opened the Chong Do Kwon North Region School in Uijongbu in 1965. Received his Jr. Master degree certification in 1969 and became the Director of the North Region TKD Schools.  Directed the 1st Jr. TKD Tournament of the North Region which was only for children of Black Belt level and had 12 divisions. Grand Master Yi took only 13 students to this tournament, they won 11 gold, 1 silver, 1 bronze.  Became the National Athletic Association TKD Officer of the North Region in 1970.  Taught at all the Elementary schools and 2 of the High Schools in Uijongbu during 1970-71 which had over 3000 children.  Received his International Instructor Certificate from the National TKD Headquarters and his Master degree of 5th Dan Black Belt in 1972. He also went to the 1st National TKD Seminar/Training, which was only for Master ranking and above.   Came to the U.S. as a TKD Instructor/employee for Kim's Academy in 1973 with an International TKD Instructor visa. Taught at Iowa State University in 1973 through 1975. Began teaching at Kansas State University in 1975 and still has his teaching Black belts there. He also opened the 1st Sun Yi's Academy of TKD at Salina. Moved to Topeka where he opened the Headquarters school in 1976. Founded/Directed the 1st Sun Yi's Academy TKD Tournament in 1980. Founded the Traditional TKD Association in 1984. Was the National TKD Tournament Director for the AAU Jr. Olympics at University of Kansas, Lawrence in 1988. Since then Grand-Master Yi has Started several branch schools throughout the U.S. Performed several demos to help fundraising for: Cornerstone, American Cancer Society, SADD and more. Instructed/trained over 700 Black Belts and taught over 30,000 students in the U.S.Now, Grand Master Yi continues to teach and train.