Steam purity is influenced not only by performance of steam separator and quality of boiler water but also by boiling condition and height of water level in boiler steam drum. Water level in boiler steam drum is substantiablly higher than the gauge level, because of the water flow in the drum and void under the water level, and it is necessary to evaluate the actual level height for the reasonable designing of drum to get optimum steam purity. This report describes the results of experiments on the boiling condition of water sarface in boiler steam drum and actual water level in relation to the steam purity. The experiments were performed in the natural circulation boiler of the same type as latest marine use. Boiling condition was observed by ITV camera from the peephole at the drum. The results show that most violent swell was observed at the drum midpoint, and boundary between steam and water at the normal condition was clear but remarkable water flow to down comer in drum end, and fog in steam space by drop of steam pressure was observed. Actual water level in the drum is measured by ITV camera method and γ-ray penetration method. Our experiments show that actual water level in the drum is 110 mm heigher than gauge level and swelling surface is 160 mm heigher than gauge level. Steam purity is messured by analizing quantitatively solid content in the steam by activable tracer method. The results show that steam quality at the drum outlet is 99.9% when steam velocity is less than 0.11m/s and the height of steam space is 300 mm.