This study investigated the various admission policies and processes used for selection of students into Nigerian Universities. The research also assessed the impact of students’ admission policies into university on students’ academic achievement in South-West, Nigeria during the 2008 to 2012 academic years. Research samples in this study were academic and faculty officers, admission officers, students and lecturers of public universities in Nigeria. The instruments used are the students’ questionnaire on admission policy and students’ academic achievement (SQAPSAA) and staff questionnaire on admission policy and academic achievement of students (SQAPAAS).The design of the study was both descriptive and correlation research designs. To carry out the study two research questions were answered using percentages, mean and standard deviation while the two hypotheses formulated were tested using analysis of variance and t-test statistics. The sample for the study consisted of 1200 students, 150 lecturers and 42 non-teaching staff randomly selected from six out of 10 public universities in South-West Nigeria using both purposive and stratified random sampling techniques. This study shows that there was significant difference in the academic achievement of students enrolled through various modes of admission. The results also indicated that there was no significant difference in the academic achievement of students who were admitted into the university through unified tertiary matriculation examination (UTME) /post-UTME test and those admitted through pre-degree programme and direct entry admissions. There was significant difference in the academic achievement of male and female undergraduates. Based on the findings of this study, it is recommended that the government should ensure strict adherence to student admission process as specified by the National Universities Commission (NUC) and allow the catchments area policy to give rise to meritocracy. The study also recommends that university authority should admit students into the university at reasonable proportion to improve academic achievement of the undergraduates. Attempt should be made to reduce the achievement gap between male and female students in the universities. Apparently, some of the policies for admission into the universities are no more relevant, hence should be scrapped. The universities admission policies in Nigeria need to be revamped and made result oriented with the aim of improving academic achievement of students after admission into university. Both JAMB-UTME and post-UTME mode of admission be used mutually for future admission exercise into Nigerian universities.