The principle of sufficient reason is one of the most significant philosophical principles. Arthur Schopenhauer, the well-known German philosopher, has emphasized on this principle and taken it as the entrance key element to his philosophical system. He tries to characterize the limits and conditions of application of this principle, and to avoid the inappropriate use of this principle outside the phenomenal reality domain. In his philosophical system, this principle governs the relations between phenomena and objects. Furthermore, the mentioned principle is not equal to the principle of causality, but it is more general than that. For the principle of causality is only one of the four forms of the principle of sufficient reason. This paper tries to discuss the content, application domain and importance of this principle in Schopenhauer׳s philosophy.