Based on the quantum theory of the intracavity polaritons, the electromagnetically induced transparency phenomenon (EIT) in a cavity is investigated experimentally. By increasing the temperature of the rubidium vapor cell, a substantial increase of the number of rubidium atoms is achieved. When the collective strong coupling condition is satisfied, a weak control field is sufficient for the realization of EIT and significant narrow linewidth. With the further increase of the temperature, the collective coupling effect is enhanced and the cavity linewidth is further narrowed.