The objective of the research was to study the determinants affecting the decision to study nautical science program within a Thai maritime institute in preparation for working post-graduation as a ship officer on a merchant ship. The samples are classified by institute, academic year, cumulative score level, domicile, and parent's monthly income. The total sample of study was 386 Thai merchant marine students. The data collection method was a one to five rating scale questionnaire. The statistical methods applied in analyzing the data were percentage, mean, standard deviation, t-test, one way analysis of variance and a Sheffe's test. The study shows seven factors that influenced the decision in descending order; expectations, tuition and scholarships, selection system, quality of the institute, background and private capability, generality of the institute and external influences on the decision. The decision to select an institute was classified by institute and revealed that different institutes had distinct determinants that led to the decision. The students were from The Merchant Marine Training Center and from The International Maritime College, Kasetsart University were differences. There were no dissimilarity between academic year, cumulative score level, domicile, and parent's monthly income.