In Taiwan, over 800 thousand tons per year (TPY) of biowastes such as sugar cane bagasse, sugar cane leaf, rice straw, and corn leaf are produced. These biomasses are the major wastes of farms and are abundantly available. However, these biowastes cause disposal and landfill problems. The main component of these biowaste ashes is SiO2. High crystallinity (99%+) zeolites ZSM-5 and ZSM-48 synthesized from the reaction mixtures containing an amorphous silica source from ashes of these biowastes' gasification were investigated. The Si/Al ratios of synthetic zeolite products were determined by atomic adsorption, X-ray fluorescence (XRF), and induced couple plasma/mass spectroscopies (ICP/MS). X-ray powder diffraction (XRD) and scanning electron microscopy/energy dispersive spectroscopy (SEM/EDX) data indicate ZSM-5 or ZSM-48 with a high crystallinity can be obtained at a 48 h crystallization time in a high pressure ( 15 atm) autoclave at 433 K.