Zeolite L was prepared from the substrate system of Na2O-K2O-Al2O3-SiO2-H2O at temperatures of 373-443 K by hydrothermal crystallization. The influence of various synthesis parameters such as the concentration ratios of the components, starting raw materials, synthesis temperature, gel aging, and stirring on the crystallization was investigated. Investigations revealed that the crystallinity of zeolite L crystals depends on molar ratios of the components such as SiO2/Al2O3, (K2O+Na2O)/SiO2, Na2O/(K2O+Na2O), and H2O/(K2O+Na2O). Pure and highly crystalline zeolite L could be obtained from a gel with the molar composition 5.4K2O-5.7Na2O-Al2O3-30SiO2-500H2O after 24 h at 443 K. It was found that the silica source affected the crystal size of zeolite L, and as the synthesis temperature increased, the average crystal size became larger. The crystal size could be decreased significantly by stirring the gel or subjecting the substrate mixture to an aging treatment at room temperature prior to the hydrothermal treatment. Thermal stability of the zeolite L crystals obtained was also briefly investigated.