The fracture toughness of hydroxyapatite (HAP)-filled polycaprolactone (PCL) biocomposites was investigated using the technique of essential work of fracture (EWF). The influence of specimen thickness on the toughness parameters was investigated. The specific essential work of fracture (we) was found to decrease with the increase in filler loading and with the thickness of the specimen. The testing procedure used for the EWF measurement obeyed the validity criteria of the concept and we determined can be considered to be a good measure of the plane-stress toughness of the composite. The adhesive strength between the HAP and the PCL phase was determined using T-peel tests. The layer of the laminates in the PCL–HAP–PCL peel test specimens was varied to study the influence of laminate thickness on the interfacial work of fracture. The effect of temperature on the interfacial work of fracture was also evaluated. The parameters were varied to determine the effect of the plastic deformation on the peel strength. The interfacial work of fracture between HAP and PCL was found to be relatively strong; however, the effect of temperature and PCL laminate thickness did not significantly vary the adhesion strength.