The structure-directing effect of C18H37-N+(CH3)(2)- C6H12-N+-(CH3)(2)-C6H13(Br-)(2) surfactant (C18-6-6) for the formation of MFI zeolite nanosheets was investigated under various synthesis conditions containing a small amount of C18-6-6 in the presence or absence of Na+. Each synthesis mixture after heating for a different period at 413 K was analyzed by X-ray powder diffraction, electron microscopy, and argon adsorption. The synthesis with Na+ yielded a small amount of 2.5 nm thick zeolite nanosheets at an early reaction time, while most of the silica source remained as an amorphous phase. As the reaction time increased, the thickness of the zeolite nanosheets gradually increased, along with the depletion of the amorphous silica phase. When Na+ was not present, the initial synthesis result also showed a small amount of 2.5 nm zeolite nanosheets, but the zeolite thickness did not change since then on. Hence, the C18-6-6 surfactant was able to rapidly generate the 2.5 nm zeolite nanosheets and, subsequently, Na+ ions gradually participated in the structure directing process to increase the zeolite thickness. By properly balancing the structure-directing effects of C18-6-6 and Na+, it was possible in the present work to control the thickness of the MFI zeolite nanosheets systematically (e.g., to 20 nm).