A real space ab initio molecular dynamics method reinforced by the double grid technique has been applied to the simulations of the reactions of OH radical/OH anion with formaldehyde to examine their reactivities and also to test the efficiency of the method. The simulation revealed that the H-abstraction reaction by the OH radical takes place without potential energy barriers within the level of generalized gradient approximation (GGA). The heat of the reaction computed by a gradient-corrected functional is 33.3 kcal/mol, which is in good agreement with the experimental value of 33.2 kcal/mol. The molecular dynamics of the formation of tetrahedral complex [HCHO-OH](-) in the ionic process has also been found. The optimized structure of the complex was compared with those obtained by MP2 and BLYP calculations by Gaussian 98, which showed excellent agreement.