The existence and uniqueness properties of arc-length orthogonality and the existence and unique-ness of orthogonal diagonals in the sense of arc-length orthogonality are studied in normed linear spaces． By apply-ing related results of homogeneous direction of isosceles orthogonality ( Pythagorean orthogonality，resp． ) ，the rela-tion of arc-length orthogonality to isosceles orthogonality ( Pythagorean orthogonality，resp) is studied． It is provedthat if isosceles orthogonality ( Pythagorean orthogonalityresp． ) implies arc-length orthogonality or arc-length or-thogonality implies isosceles orthogonality ( Pythagorean orthogonality，resp． ) in the intersection of a neighborhoodof a fixed unit vector and the unit sphere in a real Banach space，then the underlying space is an inner product space．