a screw head having a top surface, and an inner surrounding surface that is connected to said top surface, said inner surrounding surface defining a recess that is recessed from said top surface, said inner surrounding surface having six curved portions and six driven portions that are disposed about a central axis of said fastener in an alternating arrangement, each of said driven portions having two opposite ends that are respectively connected to two of said curved portions adjacent to said driven portion, each of said driven portions having a curved section that is connected to one of said two of said curved portions and that has a direction of curvature opposite to said one of said two of said curved portions, a straight section that extends along a tangent line of an end of said curved section distal from said one of said two of said curved portions, and an extension section that is connected between said straight section and another one of said two of said curved portions; and

a screw shank extending downwardly from said screw head, an outer surrounding surface of said screw shank being formed with a thread that helically extends along said screw shank;

wherein an intersection formed between said extension section and said straight section of each of said driven portion is configured to be in a form of an acute angle, a right angle, an obtuse angle or a fillet;

wherein a radius of curvature of said curved portions is greater than said curved sections of said driven portions, and the radius of curvature of said curved sections of said driven portions is greater than a fillet radius of said intersection formed between said extension section and said straight section of each of said driven portion; and

wherein an osculating circle of said curved section of each of said driven portions is simultaneously tangent to osculating circles of said two of said curved portions adjacent to said driven portion.