The yielding behavior of epoxy-amine thermosetting resins was investigated by a study of the stress-strain curves in compression mode, recorded at various temperatures and strain rates. The materials under study included pure resins of variable chain flexibility and cross-link density, as well as dense resins modified by the addition of anti plasticizers in the initial monomer mixture. Two types of antiplasticizer were examined, depending on whether they remain miscible to the network or give rise to nano-scale phase separation along network construction and lead to materials of improved toughness. Data analysis was performed on the molecular scale, by putting emphasis on the correlation existing, as in thermoplastic amorphous polymers, between the yielding and segmental mobility associated to the beta-relaxation processes.