The potential energy surface for the reaction of C(P-3(j)) with ethylene, C2H4((XAg)-Ag-1), has been investigated using the G2M method. Structures of the stationary points and transition states for various isomerization and dissociation pathways of triplet C3H4 have been studied. The results show that at the initial step of the C(P-3(j)) + C2H4(X(1)A(g)) reaction carbon atom attacks the pi -orbital of the C2H4 molecule yielding cyclopropylidene ii without entrance barrier, ii then isomerizes to the triplet allene i3 via ring opening. The latter either splits a hydrogen atom producing the propargyl radical p3 + H or undergoes a 1,2-H shift to vinylmethylene, which in turn gives H2CCCH (p3) + H. The propargyl radical is concluded to be a nearly exclusive product of the C(P-3) + C2H4 reaction. At the internal energy of 9.2 kcal/mol above the reactants level, Rice-Ramsperger-Kassel-Marcus calculations show about 93% of H2CCCH comes from fragmentation of triplet allene and 7% from vinylmethylene. The formation of CH2(B-3(1)) + C2H2 via the vinylmethylene intermediate gives only 2% of the reaction products; the formation of triplet C3H2 + Hz is unlikely. This study completes a comprehensive investigation of the C(P-3(j)) + C2H4 reaction; its rate constants have been measured in a broad temperature range from 10 to 800 K, the reaction dynamics has been unraveled by crossed molecular beam experiments, and the reaction potential energy surface has now been explored by ab initio calculations.