A series of epoxy networks with controlled molecular weight between crosslinks (M-c) was constructed with a difunctional epoxy resin and a mixture of aliphatic amines. The glass transition and yield strengths decreased as M-c increased, while the elastic properties were independent of M-c. In determining the effect of M-c on fracture toughness, the fracture behavior changed from a brittle fracture to a ductile fracture as M-c increased. The tests and analyses used to evaluate the fracture energy changed from a linear elastic fracture mechanics approach for brittle failures to an elastic-plastic fracture mechanics approach for ductile failures. The ductile responses also shaved an increasing fracture resistance with crack extension. Two popular models were used to describe the fracture energy as a function of yield strength. The analysis showed that a change in fracture energy by a thermally induced change in yield strength was equivalent to a change in fracture energy by a chemically induced change in yield strength. In addition, comparisons between the two models allowed insightful relationships to be drawn.