Mathematics expresses itself everywhere, in almost every facet of life in nature all around us and in the technologies in our hands. Mathematics is the language of science and engineering describing our understanding of all that we observe. It is an attempt to find out the sources that are in the students which keep them with interest in learning Mathematics with love and circumstances which helps them to learn Mathematics in order to achieve high marks. Language and mathematical skills are very much essential for the students in all walks of life. The present study examined the achievement in mathematics and mental alertness of higher secondary second year students. It was administered to 800 Higher Secondary students from 12 schools around Nagapattinam Educational District. In the findings, the researcher arrived at the result that Achievement in Mathematics and Mental Alertness of higher secondary students is average. There is significant positive relationship between Achievement in Mathematics and Mental Alertness of higher secondary students. Further Mental Alertness has a very strong association with Achievement in Mathematics of higher secondary students.