Fatigue crack propagation behaviors of epoxy adhesives under mode I loading were investigated. Unmodified and rubber modified epoxy adhesives (Shell Epikote 828) were used while high strength carbon steel was used as adherends. In order to know the relationship between crack growth and surface conditions of adherends, adherends were treated by three different methods: acetone cleaning only, sulfuric acid and chromic-sulfuric acid treatments. Experimental results revealed that the fatigue crack growth resistance of epoxy adhesives greatly increased due to acid surface treatment of adherends. In all cases of surface treatments, a mixed failure mode was observed. The failure mode transfers from cohesive failure to adhesive failure with decreasing δG1. It must be noted that very interesting phenomena for crack propagation were observed in both cases of sulfuric and chromic-sulfuric acid treatments. When the test was conducted at a constant amplitude of cyclic displacements, the fatigue crack velocity decreased and the crack was arrested once with decreasing δG1 (δG1=70∼80 J/m2). However, the crack began to propagate again. Then, the crack finally met the threshold which was appreciably higher than that for acetone cleaning. This phenomenon does not appear under the dry environment as well as in case of acetone cleaning.