The strengthening and grain boundary embrittlement in an age hardenable Fe-20.4Ni-2.8Ti ternary alloy were investigated. The Vickers hardness and tensile properties were evaluated using a Vickers hardness and tensile tester and the precipitation behavior during aging treatment was observed by transmission electron microscopy (TEM). The fracture surface was observed using low voltage field emission scanning electron microscopy (FE-SEM). The alloy showed typical aging hardening curves with a single aging peak near 640 HV, but was found to undergo severe grain boundary embrittlement from the initial stages of aging treatment. Many fine particles were observed at the grain boundary fracture surface. These particles were identified as η-Ni3Ti precipitates nucleated at the prior austenite grain boundaries. When the aging time was extended, austenite nucleated at the interface of the matrix and η-Ni3Ti precipitate. With the formation of the austenite, the tensile ductility was recovered. It was concluded that the precipitation of the η-Ni3Ti intermetallic particles at the prior austenite grain boundaries and formation of the austenite are the main causes of embrittlement and subsequent de-embrittlement in aging of this alloy.