Large petroleum storage tanks are vulnerable to internal pressure due to their sensitivity to imperfections. Blast pressure can cause imperfections, which increases storage tanks vulnerability to collapse. Collapse of tanks can put surrounding area at risk of fire and explosion. Hence, correct load determination and analyses are important. Correct distribution of the blast loading is important for storage tanks containing hazardous materials, as it is the local load acting on structure, especially for thin walled structure. This paper gives results of a series of analysis performed on STAAD-Pro on the distribution of blast pressure on the walls of large storage tank with different liquid filled condition. Using regression analysis, expressions that fit the analysis results very well were derived and critical filled condition of the tank was determined.