Frequentist Probability concepts were introduced and much of probability mathematics derived, classical statistical inference methods were developed, the mathematical foundations of probability were solidified and current terminology was introduced. This article describe about Frequentist Probability, which is the body of knowledge that enables to reason formally about uncertain events. It defines an event’s probability as the limit of its relative frequency in a large number of trials. Frequentist Probability are discussed only when dealing with well-defined random experiments or random samples. The set of all possible outcomes of a random experiment is called the sample space of the experiment.