The aim of the current research is to identify the nature of the correlation between mental fitness and the depth of knowledge in mathematics among intermediate school students, by answering the following question: What is the nature of the correlation relationship and its type, direct or inverse, between mental fitness and levels of depth of knowledge in mathematics for secondary school students, according to the gender variable? The two researchers adopted the descriptive research approach, the method of studying relationships and their connection, and to achieve that goal, a group of zero hypotheses, which are seven hypotheses, were developed. The number of students of the research community was (10210), with (4763) male students and (5447) female students, and the research sample consisted of (385) students, divided into (181) male students and (204) female students from the fourth scientific grade in secondary schools. And the secondary school affiliated to the General Directorate of Education of Baghdad / Al-Karkh First. For the purpose of collecting data for the research, a scale of mental fitness was built for fourth science students, which consisted (in its final form) of (30) test items, and a test of the depth of knowledge in mathematics for fourth scientific students. This included (37) items with (16) items of the essay type, and (21) items of the objective type (multiple choice), after analyzing the contents of the fourth grade book, first semester, thirteenth edition, for the academic year 2022-2023, according to the indicators of the depth of knowledge in mathematics that were prepared by the two researchers. The clarity of the scale items, the test items, the instructions, and the time required to conduct the scale and test were verified. Appropriate statistical analyzes were conducted for both the mental fitness scale and the test for the depth of knowledge in mathematics to find out (discrimination coefficient, coefficient of difficulty and ease of the test items of depth of knowledge in mathematics and the discriminatory power of items of the mental fitness scale), as well as verifying the psychometric characteristics of them. After the statistical tools were chosen to analyze the results of the application of the two tools, such as the T-test for one correlated sample, as well as the T-test for two independent uncorrelated samples, Pearson correlation coefficient, the alpha Cronbach equation, and the T- equation to measure the significance of the correlation, the results indicated that there is a strong positive correlation between fitness and the mentality and the depth of knowledge in mathematics among the students of the fourth scientific grade, which is a direct relationship. Finally, some recommendations and suggestions were made.