The interaction of two Laguerre-Gaussian(LG) modes carrying opposite topological charges in a linear medium is investigated experimentally. The changes of diffractive pattern are observed when these two LG modes propagate, and a dark arc happens by the central dark particles of the two LG modes joining together in case the interaction of two LG modes carrying opposite topological charges propagate in a linear medium. This experimental phenomenon proves that the screw phase dislocations evolve into a new type of dislocation—edge phase dislocation with the optical field propagating.