The cell size variation in overdriven gaseous detonations is studied in hydrogen/oxygen and acetylene/oxygen mixtures.The local self-similarity of Mach reflection of detonations on the wedge in the far field renders the presence of a steady overdriven Mach stem to be possible.The study focuses on the cell size change of overdriven Mach stem on the wedge surface other than on the sidewall.The detonation cell pattern on the wedge surface has a complicated process of three-stage pattern,i.e.,the cells decreasing from large to small size,and then increasing asymptotically to a medium size and keeping constant.The cell size ratio with increasing the degree of overdrive is also examined.It is found that the ratio decays as the degree of overdrive increases.However,as the wedge angle increases to a critical value,finer cells are not created on the smoke foils.Ng\'s model used to predict the cell size is also found to be valid only for detonations with relative large instability parameters,but presents large errors for highly overdriven detonations with low instability.A modification to Ng\'s model is proposed based on the experimental results.