In a medium bowl, combine the sugar, fish sauce, soy sauce, vegetable oil, garlic, black pepper and red pepper flakes. Mix until the sugar has dissolved.
Place the chicken in a shallow dish and pour the marinade over it. Cover and refrigerate for at least 30 minutes.
Heat a large skillet over medium heat. Add the chicken and marinade to the skillet and cook for 8-10 minutes on each side or until cooked through.