Flame speeds of all hydrocarbon fuels are very sensitive to the reactions of formaldehyde (CH2O) and formyl radical (HCO). In this paper, we studied the prompt dissociation effect of HCO on the flame speeds of trioxane and formaldehyde up to 9 atm. The effects of prompt HCO dissociation were examined at high pressure. Different kinetic models were compared against the experimental results. The results showed there was a significant effect of the HCO prompt reaction on the flame speed in a broad range of pressure and equivalence ratio. At higher pressure, it was observed that the prompt effect was reduced due to the third-body collision effect. However, with the increase of O2 mole fraction, the prompt effect was enhanced. Comparison showed that HP Mech with the inclusion of the prompt reactions had a better prediction at lean and ultra-lean conditions than other mechanisms. The improved prediction of the flame speed indicated that the prompt effect needed to be considered in the model development of hydrocarbon and oxygenated fuel combustion.