The girl child participation in primary school level of education has become a real concern in all nations of the World particularly in the developing countries of the Sub-Saharan Africa, Kenya included. Girl child education worldwide provides benefits to the family and the society at large thus factors affecting it should be addressed for the sake of human and societal development. The objective of the study was to establish the factors affecting girls’ participation in primary schools in Kenya. Specifically, the study investigated the effect socio-economic, socio-cultural and school -based factors have on a girl-child’s participation in primary school education in Kenya. In some parts of Kenya, there is low girls’ participation in primary school education thus the interest in establishing the factors that may be contributing to this problem. This was considered crucial as Kenya strives to achieve basic Education for All by 2015. The study employed a survey design. 210 girls drawn from primary classes 6, 7 and 8 participated in the study. The simple random technique was used in the selection of the sample. A questionnaire was used to collect data that were analyzed by use of descriptive statistics (frequencies and percentages). The findings of the study revealed that socio-economic, socio-cultural and school related factors affect girls’ participation in primary schools in Kenya. In effect, unless these factors are expeditiously addressed, primary school girls will continue to be disadvantaged in the Kenya’s school system. This will frustrate the world’s efforts in attaining basic education for all by 2015. The study therefore recommended that the Ministry of Education in Kenya should create awareness to all education stakeholders on the importance of girl child education in the country. It was also recommended that a large scale study involving sub-Saharan Africa developing nations experiencing low girls' participation in education be carried out to falsify or corroborate the findings. Key words: education, factors, girls’ participation, socio-economic, socio-cultural, school-based.