In reality, it is often found that some people who have no school experience show sophisticated understanding of number relations and operations. This phenomenon raises a question of how they think and view toward numbers, operations and their relationship in a number system. This understanding will provide us information about the nature of human cognitive development about numbers that is not affected by school programs. Therefore, this study intends to address these questions. The study focuses on investigating illiterate counting strategy in solving mathematics problem in four basic mathematics operations, such as addition, subtraction, multiplication and division. The study involved two illiterate participants whose profession as merchants in traditional markets. They were given some mathematics problems in contexts and non-contexts relating to the four basic operations and then are interviewed intensively and deeply in order to clarify their reasoning and thinking in solving the problems. The findings show that illiterate people are able to perform relatively well complex mental mathematics which is mostly based on number senses. Comparing to schooling people who solve mathematics problems by mostly applying a common algorithm, procedure or formula, illiterate people developed their own strategy which is mostly based on their understanding of relations of numbers (number sense). They are able to utilize splitting, compensation, familiar numbers, place value system, doubling, ratio, part-whole relationship, commutative, associative, and distributive property as their counting strategy. It is also found that out-of-school environments, such as cultural or fieldwork practices, mainly contribute to illiterate numeracy development. It is lead to the claim that while most literate people acquire numeracy acquisitions during school days, illiterate people spend their time outside learning other types of skills that shape their cognitive strategies in a way quite distinct from someone who has school experience. The findings of the recent study can be the consideration for teachers and policy makers in designing learning activities and tasks as well as mathematics education curriculum.