Bring large pot of lightly salted water to a boil. Add green beans; cook until crisp-tender, about 8 minutes.
Drain; run under cold water or submerge in ice bath until well chilled. Drain; shake off excess water. (Can be made up to this point a day ahead. Cover and refrigerate.)
Heat oil in large skillet over medium heat. Add walnuts; stir until lightly browned. Add beans, garlic, salt and pepper; heat through.