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The purpose of this research was to study mathematical process skill ability of elementary students. The purposively selected sample consisted of 461 Prathom Suksa 4 students from six schools in Bangkok and six schools in the provinces during the academic year 2015. The employed research instrument was a mathematical process skill test with KR-20 reliability coefficient of 0.73, the difficulty indices of the items ranging from 0.28 – 0.78, and the discriminating indices ranging from 0.26 – 0.65. Data were analyzed by a computer program. The research results showed that 63.12 percent of students in the sample had mathematical process skill ability at the poor level (having less than 50 percent of full score).