In this study, fatigue reliability of halogen free substrate noticed as an environmental harmony type printed circuit board backing was evaluated. And, the examination was demonstratively carried out on the difference of failure mechanism in fatigue breakdown and static destruction from the mesoscopic viewpoint. (1) There was a difference in both fatigue life, though mechanical property of halogen free material was almost equal to that of halogen material. That is to say, the fatigue life of halogen free material was longer than that of halogen material. (2) In halogen free material, the crack was comparatively generated in initial stage in fatigue life. In the meantime, in the halogen material, it immediately came to rupture, when crack was generated, and it was frailly destroyed. (3) From the fractographic study, it was indicated that the difference in fatigue life of halogen free material and fatigue life of halogen material was controlled by the difference of size (Unit Fracture Area) of the characteristic destruction pattern. (4) In fatigue breakdown, matrix resin is a weakest link. In the meantime, the effect of the difference of matrix resin does not appear in the static destruction in the sake glass fiber strength (that is to say, it is a strongest link).