The crystal size of ZSM-5 zeolites synthesized through the hydrothermal synthesis with commercial silica sol could be controlled by adding colloidal silicalite-1 seeds. Different-sized silicalite-1 crystals were synthesized uder different temperature and crystallization time by clear liquid reflux method. After centrifuging, washing and drying, the properties of the solid silicalite-1 crystals were characterized. The results of  XRD and IR indicated that both the seeds possessed MFI structure.  The crystallinity of the seed increased with the increase of crystallization temperature. The particle size of silicalite-1seeds depended on the synthesis temperature when other synthetic conditions were the same.  The ZSM-5 zeolites with controllable crystal size in the range of 200 nm-2200 nm could be obtained by adding the silicalite-1 seeds with different amounts and synthesis conditions. The growth of ZSM-5 crystal for the seed-induction synthesis process followed a seed surface crystallization mechanism.