a = [[9, 9, 2], [9, 9, 3], [9, 9, 8], [9, 9, 4], [9, 9, 1], [9, 9, 5]] b = sorted(a, key=lambda a:a[2]) #b contains the following arrays, sorted by their second index: [[9, 9, 1], [9, 9, 2], [9, 9, 3], [9, 9, 4], [9, 9, 5], [9, 9, 8]]