ABSTRACT For many years academicians and researchers have debated about factors that influence a student academic performance in mathematics. From this debate, they have come to agree that soclo- economic factors are among other factors that influence a student’s academic performance in mathematics. This study aimed at establishing socio- economic factors that affect mathematics performance of students at Kenya certificate of secondary education of St. Paul’s Igonga secondary school in Kisii central district, Kenya. Objectives of the study were to: determine the profile of the respondents, the performance in mathematics at KCSE level at St Paul’s Igonga Secondary School for the last three years, Students performance in mathematics at KCSE level. Socio economic factors affecting students’ performance in Mathematics at KCSE level and determine if there is a significant relationship or difference between socio economic factors and students’ performance in mathematics at KCSE level. Questionnaires were used to source information on the social economic factors influencing mathematics performance. Data was analyzed using Software Package for Social Sciences (SPSS ® version 11.5). Most of the student respondents were males indicating gender bias in enrolments whereas many students were between the ages of 15 to 17 years. Entry behavior in mathematics from primary school was high (B). The study indicates that parental economic activities, fees payment responsibility having both parents give frequency of being sent home for fees and availability of instructional materials are highly significant (P< 0.05) to students’ math emetics performance in mathematics at K.C.P,E level and thus could imply the same at K.C.S.E. Therefore it is recommended that the government of Kenya should allocate more bursaries to enable students learn because majority are often sent home for fees and are from single parent family hence strain financially leading to frequent absenteeism of students due to lack of school fees.