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Silicalite-1 membrane was prepared on an outer surface of a tubular α-alumina support by a secondary growth method. Changes of defect amount and crystallinity during secondary growth were carefully observed. The defect-less well-crystallized silicalite-1 membrane was obtained after 7-days crystallization at 373 K. The silicalite-1 membrane became (<i>h0l</i>)-orientation with increasing membrane thickness, possibly because the orientation was attributable to “evolutionally selection”. The (<i>h0l</i>)-oriented silicalite-1 membrane showed high <i>p</i>-xylene separation performance for a xylene isomer mixture (<i>o</i>-/<i>m</i>-/<i>p</i>-xylene = 0.4/0.4/0.4 kPa). The <i>p</i>-xylene permeance through the membrane was 6.52 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ with separation factors α<i>_p/o</i>, α<i>_p/m</i> of more than 100 at 373 K. As a result of microscopic analysis, it was suggested that a very thin part in the vicinity of surface played as effective separation layer and could contribute to high permeation performance.