Her breakthrough year was 2006. On 24 March 2006 she won the women's 800 m event at the 2006 Commonwealth Games with a time of 1 minute, 57.88 seconds. She managed to beat veteran runner Maria de Lurdes Mutola of Mozambique. Later same year she won African Championships in Athletics at the 800 m event. She also beat Kenyan 800 m women's record twice. Jepkosgei was awarded the 2006 Kenyan Sportswoman of the Year award.