Abstract: The adsorption and reaction of H_2CO on oxygen covered Ag(110) in ultrahigh vacuum were studjed with thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED) and electron stimulated desorption ion angular distribution (ESDIAD). For the oxidation of formaldehyde, the TDS, LEED and ESDIAD results indicate that interaction of formaldehyde with oxygen on Ag(110) is quite complex. Each oxygen adatom can stabilize several molecules of formaldehyde, most of which desorb as H_2CO at 220 K. Furthermore, the ESDIAD photos taken after annealing to 225K suggest the presence of H_2CO_(2(a)) intermediate which dehydrogenates to produce a reaction-limited H_2 desorption state and to give adsorbed, ordered formate (HCOO_(a)) and H_2(2) above 235K.In addition, second H_2CO desorption peak and H_2O peak were found at about 250K. The adsorbed formate (HCOO_(a)) completely decomposes into CO_2 and H_2 at about 395K.