Stress analysis was conducted for a traversely reinforced composite plate with an interface crack at the center under tension in plane strain condition by the boundary element method (BEM). The results obtained are summarized as follows ; (1) When the normalized crack length, a=a/d, (a : crack length, d : thickness of matrix layer) is much smaller than unity, the magnitude of stress intensity factors (SIFs), K₁, K₂, and K_i=(K²₁+K²₂)^1/2, are close to the SIFs for an interface crack in a dissimilar infinite body. The crack is termed as the small crack for composite materials. (2) As a becomes larger, the normalized SIF, K_i decreases and shows the minimum near a=1, where K_i is the SIF, K_i, divided by the SIF for a homogeneous orthotropic body composed of a Mode I crack, K₁h. It increases and converges on a constant value as a increases. (3) The energy release rate calculated from the convergent K_i coincides with that of the crack in a homogeneous orthotropic body where the elastic constants are determined by a mixture of those in the matrix and fibers. The crack is termed as the large crack for composite materials. (4) Since the region of oscillated stress distribution is confined in the vicinity of the crack tip, the stress intensity is represented by K_i, of which values are nearly equal to the SIF of the large crack in a homogeneous orthotropic body.