Surface defects in structural members are apt to be origins of fatigue crack growth, which may cause serious failure of whole structures. In real structures, defects are often found rather in multiple clustered configuration at some geometrically discontinuous parts of the structures, such as welded joints, corner parts, etc. Analysis on crack coalescence and growth from these multiple defects, therefore is one of the most important subjects for the safe-life estimation of structures. The authors have performed an experimental analysis on fatigue crack propagation from two colinear surface defects. Mutual interaction effects of cracks are discussed using a simplified formula on crack propagation rate, together with a trial analysis on two-dimensional cracks. Changes in crack shape during propagation are well characterized by analogy with a single surface crack model. A set of formulas is presented on the prediction of crack length and life at the point of crack penetration of plate thickness.