This research study investigated the impact of Pentecostalism on the Orthodox Churches in Northern Nigeria (1976-1996). The interest of the researcher on this subject arose from the numerous constant criticisms leveled against the Pentecostal movement. They are accused of commercializing Christianity, over-simplifying the truth of the Gospel, preaching Christianity without theology et cetera. The study examined the growth, strategies of expansion and effects or impact of Pentecostalism on the Orthodox Churches in Northern Nigeria. Seven null hypotheses were formulated for the study. The data gathered through questionnaire were analyzed using frequency, percentages and T - Test as appropriate. The statistical tests revealed that three of the seven null hypotheses were retained while four were rejected. The instruments used for the collection of data for this study were questionnaire and observation. A total of 900 copies of the questionnaire were sent out and administered to ministers and members of both Pentecostal and Orthodox Churches of which 798 were returned. Of these, 720 were found to be usable for the research. The researcher personally observed three Pentecostal Churches and three Orthodox Churches. The findings of the research revealed that:1. There was a rapid growth of Pentecostalism across the Northern States of Nigeria from 1976 - 1996. 2. The factors responsible for the rapid growth of Pentecostalism include: the crave for materialism, spiritual malnourished of members of the Orthodox Churches, deeper understanding of the Bible, loose living (sin) and disbelief in the spiritual gifts by some leaders of the Orthodox Churches and the search for miracles. 3. Both Pentecostal and Orthodox Churches used similar strategies for the expansion of their Churches - such as evangelism, crusades, advertisement on Radio and Television houses, demonstration of love among members and charismatic leadership of the Church leaders. 4. The Pentecostal Churches influenced many Orthodox Churches in a number of ways, e.g.: regular prayer meetings, all-night prayer, shouting "Praise the Lord, Hallelujah," drumming and clapping of hands, singing and dancing during Sunday and other worship services, casting out of demons, laying of hands, giving testimonies of the goodness of God. 5. The infiltration of Pentecostal tendencies into the Orthodox Churches has influenced the style of prayer, mode of worship, music and even administration of the Church affairs among the Orthodox. 6. The impact of Pentecostalism includes spiritual awareness among Christians, relying on prayers, willingness to give sacrificially for the sake of the gospel.