Trim the chicken thighs and flatten them out so that they are a fairly uniform thickness.
Add the sugar, salt, pepper, fish sauce, lime juice and oil in a bowl large enough to hold the chicken thighs. Add the chicken and let marinate and spoon the marinade around so the chicken is coated. Cover with plastic wrap and marinate for half an hour at room temperature.
Grill over medium heat about 6-10 minutes total, turning a few times.
Remove from the heat and let rest 10 minutes before slicing across the grain.