the present invention relates to a process for preparing nicotinic acid , which comprises directly subjecting a mixture of 3 - methylpyridine , oxygen , and water to a vapor phase oxidation in the presence of a catalyst consisting of , as active ingredients , vanadium oxide and transition metal oxide both of which are supported on a carrier , to give the nicotinic acid , wherein crystal size of the active ingredients on the surface of the carrier is controlled in a range of from 40 to 200 nm through use of transition metal oxide . the present invention further relates to a catalyst used in the oxidation . in the method according to the present invention , the nicotinic acid can be obtained in a higher conversion and a higher selectivity . therefore , the manufacturing cost can be reduced .