Statistics for Islam in Sierra Leone estimate a Muslim population of 3,610,585, representing around 60 percent of the country's total population. There are 18 ethnic groups in the country, the two largest being the Temne and Mende. The Temnes are the main tribe in the north and are predominantly Muslim.

In the early 18th century Fulani and Mande-speaking tribesmen from the Fouta Djallon region of present-day Guinea converted many Temne of northern Sierra Leone to Islam. During the period of British colonialization, the spread of Islam was indirectly favored by the spread of peace and order and stimulating trade. The Europeans broke down the traditional structures of society, introduced new educational and social ideas, opened markets, attracted migrant Africans to growing cities, and thereby indirectly promoted Islam as a form of authority and as an expression of cohesion among displaced peoples. Islam also continued to spread after independence in 1961; in 1960 the Muslim population was 35 percent and grew to 60 percent by 2000.

Source : http://en.wikipedia.org/wiki/Islam_in_Sierra_Leone