This paper deals with the problem tlf scattering of an in-plane wave by a periodic array of collinear cracks in an elastic solid. The work is of interest for the detection of cracked zone in both quantitative evaluation of materials and seismology.Scattering by elastic waves, both normally incident anti-plane longitudinal waves, by a Griffith crack was studied by Mal [1]. An integral transform method was used In (1]. Van der Hijden and Neerhoff [2] also investigated the problem of scattering of in-plane longitudinal and transverse waves by a Griffith crack, and derived a set of integral equations for the unknown crack-opening displacements. The Chebyshev polynomial expansions of the unknown crack-opening displacements were used to solve these integral equations.In recent years, scattering of waves by a periodic structure has been intensively studied for the application in the quantitative nondestructive evaluation of materials. Scattering of scalar waves by a periodic array of screen was treated in [3]. An integral equation was derived for the unknown jump of the scalar potential across the screen, and this equation was again solved by an expansion in Chebyshev polynomials of the unknown jump. The boundary integral equation method has been used to solve the threedimensional problem of reflection and transmission of a longitudinal wave by a doubly periodic array of spherical cavities [4]. The same method has been also used to solve the problem of scattering by a periodic array of inclined cracks [5], where the scattering by a periodic array of collinear cracks has been treated as a special case. The problem of scattering of waves by a periodic array of collinear cracks was first treated by Angel and Achenbach (6,7], where it was solved by using Fourier series expansion of the GreenLame potentials. In this paper, we will solve the same problem by extending van der Hijden and Neerhoff's treatment [2] of in-plane wave scattering by a sigle crack. The merit of our treatment over Angel and Achenbach [6, 7) will be discussed.