The product branching fractions for the reaction of atomic oxygen with ethene, O(3P) + C2H4→ CH3 + HCO (1a), → H + CH2CHO (1b), → H2 + CH2CO (1c), have been investigated at room temperature (295 ± 4 K) and pressures from 1 to 4 Torr (with N2 or He buffer) by a laser photolysis-photoionization mass spectrometry method. From the yield of CH3 radical, ϕ(CH3), the branching fraction for (1a) was determined to be 0.53 ± 0.04 and no apparent pressure dependence was found from 1.5 to 4.0 Torr (N2 buffer). The ratio of the HCO yield to that of CH3, ϕ(HCO)/ϕ(CH3), was measured to be less than unity and increased as pressure increased (∼0.7 at 1 Torr and ∼0.9 at 4 Torr [He]) suggesting prompt dissociation of the hot HCO radical (to H + CO) formed by channel (1a) at low pressures. An interpretation which reduces the large discrepancy among branching fractions reported for low pressure region is presented. The existence of the molecular H2-elimination channel (1c) was confirmed. The branching fraction for channel (1c) was determined to be 0.019 ± 0.001 by the yield of CH2CO and was independent of pressure from 1.0 to 4.0 Torr (He buffer). As a side result, the yield of CH3 radical from O(1D) + C2H4 reaction was also determined.