The cross section of a surface crack at the center of a crack is a single edge cracked plate. Crack opening displacement of a surface crack at center can be modified to a single edge crack with springs of which gauge length is zero and which correspond to center cracked plate. The center cracked plate can be represented by series springs for cracked part and solid part. From above relations, we calculated analytically the crack opening displacement which may corresponds to that for a surface crack by using reciprocal principle. K value at the deepest point of a surface crack can be obtained by using the solution of working stress in series springs. Then we calculated K value by series springs model for a semi-elliptical surface crack with various crack sizes in a plate subjected to tension or bending loading. These results are in good agreement with published K values.