As one of the mathematics objects, the basic facts of mathematics are the primary material that students must master. The facts of addition and subtraction should have been taught in the first level and mastered by the end of the second level. The multiplication and division facts should have been taught at the third level and could be mastered at the fourth level. The primary fact mastery phase consists of a counting phase, a reasoning phase, and a mastering/advanced phase. Mathematics as science should also be accepted by all students regardless of their characteristics, background, or physical needs. They must have the opportunity to learn and be supported to learn mathematics, one of which is a child with special needs slow learner. This research aims to describe the mastery of basic math facts in slow learner children. This is qualitative research, with research subjects totaling three slow learner students of Melana Junior Hight School, Semarang. Subjects are selected by purposive sampling. Data are collected through tests. Time triangulation is used for data validation. Data collection is carried out three times with a gap of 2-3 weeks. The data analysis technique in this research is data reduction, data presentation, and concluding. The research results conclude that the slow learner children are not yet proficient in mastering the basic facts of mathematics. There are slow learner children who can reach the reasoning stage in mastering basic facts, but more are still in the counting stage. Slow learner children who have good basic fact skills have better grades in mathematics. The addition facts are the most effortless facts to master, while the division facts are the most difficult facts to master. Some students can master multiplication facts better than subtraction facts, but some can master subtraction facts more than multiplication facts.