The ponderomotive effect of electromagnetic wave fields is practically very important in such cases as RF stabilization, RF-induced transport in magnetically confined plasma and so on. However, theoretically there are still discrepancies among the expressions derived by a number of authors. In this short note, attention is restricted to the discrepancy between fluid and kinetic expressions of the ponderomotive force often found in literatures dealing with warm plasma, and where it stems from and how it is resolved are discussed. The ponderomotive force density of electromagnetic wave fields based on Vlasov theory has been obtained. Also the fluid ponderomotive force density is given. The case of electrostatic high frequency waves in a unmagnetized plasma is considered. The discrepancy between both expressions was found. This discrepancy in a warm plasma is due to the lack of a wave-induced contribution to the thermal pressure in the fluid description of ponderomotive force density. The fluid description of the ponderomotive force can be made compatible with the kinetic one in warm plasma. (K.I.).