Combine 1/4 cup sugar, canola oil, fish sauce, chopped lemongrass, and grated garlic in a food processor or mini chopper; process until the lemongrass is finely chopped.
Transfer sugar mixture to a large zip-top plastic bag.
Add shrimp; seal and marinate in refrigerator 45 minutes.
Prepare grill to high heat.
Remove shrimp from bag; discard marinade. Thread 6 shrimp onto each of 6 (12-inch) skewers.
Place skewers on grill rack; grill 2 minutes on each side or until done.
