The second Iranian Dynasty was the Parthian Empire that ruled Iran and neighboring countries for about 5 centuries (248BC-224A.D). Not much has remained of this dynasty in Iran but two of the most important sites are Khwajeh Mountain and Kangavar Temple.
The Parthians were a group of Aryans who immigrated to the plateau of Iran and made a small independent unity. Gradually, the Parthian achieved more power. The first Parthian king was named Arsaces I and he started conquering the whole Parthia that was then ruled by the Seleucids. Arsaces and his successors still had to pay tribute to the Seleucid king until 188 B.C when Mithradates another Parthian king, started venturing further east and west and even conquered Seleucia, the capital of the Seleucids. So by this time, the kingdom further included Media, Elam, Babylon and Assyria. As the territory was so vast and a mixture of different nations and cultures, the empire had to respect their priorities in order to survive, hence the great influence of the Greeks on Parthian.
The Parthian had many wars with the Romans, but none could totally defeat the other until 110 AD when the Roman emperor, Trajan, attacked and occupied some parts of the Parthian territory and this was the demonstration of the relative superiority of the Romans over the Parthians.
Although Parthians survived this defeat and recovered parts of the lost territories, they were attacked again by the Romans in 198 AD and by this war, they moved towards the end of their reign, as in the confusion and weakness of the Parthians, a Persian vassal, Ardeshir, defeated the last Parthian king, ArtabanusV and founded the Sassanid empire.