Epoxy ternary systems were prepared from the diglycidylether of bisphenol-A (DGEBA), epoxidized castor oil (ECO), and nano-$CaCO_3$ using a thermally latent initiator. The effects of ECO and the nano-$CaCO_3$ contents on the thermal stability and mechanical interfacial properties of the prepared DGEBA/ECO/nano-$CaCO_3$ ternary systems were examined by thermogravimetric analysis (TGA), Izod impact tests, and mechanical tests. The morphology of the ternary systems was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The statistic heat-resistant index temperature ($T_s$) of the ternary systems was approximately constant up to 30 wt% ECO, and thereafter decreased with increasing ECO content. The impact strength of the ternary systems was improved by the addition of ECO and nano-$CaCO_3$. The flexural strength of the ternary systems exhibited a maximum value at 20 wt% ECO. SEM showed that shear deformation occurred, which prevented the propagation of cracks in the DGEBA/ECO/nano-$CaCO_3$ ternary systems.