This work aims to investigate the reaction mechanism of lanthanum atom with formaldehyde in the gas phase using density functional theory and coupled cluster calculations. The results indicate that the minimum energy pathway, similar to the reactions of its neighboring yttrium with formaldehyde, is the formation of the eta(2)-formaldehyde-metal complex followed by two C-H insertions which leads to metal dihydrides and carbon monoxide. The competing pathway producing a metal-carbonyl compound and hydrogen molecule favors a high-spin state and thus involves a spin conversion from doublet state to quartet state. The crossing region of the doublet and quartet potential energy surfaces (PES) has been estimated by a simple approach as proposed by Yoshizawa et al. Less favorable pathways leading to metal monoxide and carbene radical by C-O insertion as well as formyllanthanum by single C-H insertion are also studied. Compared with the CCSD(T) method, the BP86 method tends to overestimate the binding energies of the d-rich compounds, though the two methods qualitatively agree well on the reaction mechanism. Finally, the (n - 1)d(1)ns(2) to (n - 1)d(2)ns(1) promotion effect is proposed to account for the difference in the formation mechanism of the metal-carbonyl compounds LaCO and YCO, which may also extend to the reactions of formaldehyde with other "general" group III rare earth elements including Sc, Ce, Gd, and Lu.