Air bearing linear motion stages with high speed and ultra high positioning performances are always used in steppers or scanners which are the most important equipments in microlithography process. Excellent structure dynamic characteristics are the base to reach their high performances. Structure resonant frequency, as one of the most important dynamic characteristics, should always be decided in the design phase. Usually, a finite element method was used to get the resonant frequency. But air bearing linear motion stage is a fluent-structure interaction(FSI) system due to the existence of gas film, which makes it difficult to do a finite element analysis as usual. A combined finite element method in which two commercial finite element codes(MSC.nastran and FEMLAB) were used to do modal analyses of the stage design along with extensive experimental validation to evaluate stage performance. Good correlation was found between the numerical and experimental results. The results showed that the new method can be used in this kind of stages which makes designers anticipate the stage dynamic characteristics in the design phase.