The generalized planar fault energy, including the generalized stacking fault (GSF) and the generalized twinning fault energy (GTF) of body-centered cubic metal Ta are investigated based on the embedded atom potential. The GSF of Ta, much different from that of fcc metal, reveals that no evident energy minimum is observed in the energy curve. This implies that only full dislocations are possibly emitted in the 112 slip plane. From the GTF it is predicted that the minimum thickness of a metastable twin is as large as four layers and the five-layer twin is more stable. The incipient twin Ta tends to grow thicker once it is created. To confirm the significance of the GSF and GTF in revealing incipient plasticity, quasicontinuum method is used to simulate the mode Ⅱ crack of single Ta crystal. The results show that deformation twin and full dislocation along direction in 112 plane are two co-existing mechanisms of crack tip plastic deformation. The initial four-layer twin quickly extends into five-layer and more-layer twins with further loading. A full dislocation is emitted into the front of the crack tip in 112 plane. These two plastic deformation mechanisms are well explained by the GTF and the GSF respectively.