Heat oven to 350°F. In medium bowl, combine flour, pecans and brown sugar; mix well. With pastry blender or fork, cut in margarine until mixture resembles coarse crumbs. Spread in ungreased 15x10x1-inch baking pan. Bake at 350°F. for 15 to 20 minutes or until light golden brown, stirring occasionally. Cool 30 minutes or until completely cooled.