Groove pads are used quite widely in chemical mechanical polishing (CMP), and groove size plays an important role in CMP characteristics. This study focuses on the investigation of the groove size effect using X-Y groove pads which are different with pitch and width. The first experiment shows the size effect on the polishing characteristics including material removal rate (MRR), within wafer non-uniformity (WIWNU) on 4 inch oxide blanket wafers for 60 seconds. The second experiment verifies the reason why MRR and WIWNU are different, by the calculation of slurry duration time (SDT) resulting from the change of friction force. All experimental results indicated that a significant difference of slurry flow attributed to groove width and pitch has an impressive influence on friction force, finally the MRR and WIWNU are affected by the groove size.