The mechanism of the self-reaction of HCO radicals is investigated by using high-level quantum-chemical methods including M05-2X, CCSD, CCSD(T) and CRCC(2,3). Next, the rate coefficients for several product channels as a function of pressure and temperature are computed by employing statistical rate theories. Four important product channels are predicted to be CO + CO + H2, HCOH + OH, cis-(HCO)2 and trans-(CHO)2. It is found that the bimolecular rate coefficients for the formation of cis-(HCO)2 and trans-(CHO)2 are strongly pressure-dependent. The rate coefficients for the product channels CO + CO + H2 and HCOH + OH are predicted to be slightly pressure-dependent. At lower pressures and higher temperatures, the products CO + CO + H2 and HCOH + OH are dominant, while at higher pressures and lower temperatures, cis-(HCO)2 and trans-(CHO)2 formation becomes important.