Fatigue crack propagation in epoxy resins containing 0 wt% to 45wt% plasticizer was examined. In this experiment, double-cantilever type specimens were used. The stress intensity factor was adopted to express the stress near the crack tip. When specimen was cyclically loaded, not only the range of stress intensity factor ΔK but also the maximum stress intensity factor Kmax or the mean stress intensity factor Kmean were taken into consideration. Under the assumption of dl/dN=A(Kmax)α1(ΔK)α2 or dl/dN=B(Kmean)β1(ΔK)β2(dl/dN means the fatigue crack propagation rate) as the fatigue crack propagation law, dl/dN was measured for several combinations of ΔK and Kmax or Kmean and the values of constants A, α1, α2, B, β1 and β2 were evaluated from the values of log(ΔK) and log(Kmax) or log(Kmean) by means of least squares analysis.As the results, it was found that as the percentage of plasticizer increases, A and B increase by two orders, α1 and β1 decrease by 40%, and α2 and β2 decrease slightly. For the specimen without any plasticizer, both α1 and α2 are equal to about 5, and for the specimen with 40 wt% of plasticizer, both β1 and β2 are equal to about 4. It is interesting that the exponent of ΔK remains constant in spite of the decrease in exponent of Kmax or Kmean. The mechanical properties vary widely from the brittle fracture type to the ductile fracture type as the percentage of plasticizer increases, and it seems that the above interesting results about the fatigue crack propagation behavior in epoxy resins containing plasticizer are principally attributed to the difference in their mechanical properties.