It is undeniably that Prophet Muhammad (peace be upon him) is proof from Allah who has created this universe. He is the prophet who gives a grace for this world and succeeded in building peace and harmony in his era. Although he passed away but every moslem heartedly follows his Sunnah. However, moslems have different views on Sunnah, especially relating to understanding and practicing in daily life. Some of them have a radical understanding and others are not. It is sometimes caused by fanaticism, radicalism and so on. This essay tried to explore the importance of understanding on Sunnah in the beginning of Islamic history. It is absolutely to avoid a wrong understanding toward Sunnah. The writer used historical approach to answer whether Islam is radical religion or not like what some people did in the name of religion. Al-Qur'an and Sunnah from its origin, has never taught the moslems to do radicalism, terrorism, extremism and etc. But both teach moslem to be moderate and tolerance. By analyzing some historical facts, the writer found out that to have a proper undestanding on Sunnah, all moslems have to know Islamic history well, particularly how earlier moslem generation understood Sunnah properly. It is very useful for them to minimize wrong understanding and even radical understanding on Sunnah.