Background: Adolescent females have reduced ability to self-detect breast abnormalities early. Improving their knowledge and practice of breast self-examination (BSE) will reverse this trend. Objective: This study evaluated the effect of two interventions on BSE knowledge and practice among in-school adolescent females in Delta State, Nigeria. Methods: A quasi-experimental research was conducted among ninety female secondary school students selected using a multistage sampling technique from three public secondary schools in the state. There were three groups: two experimental groups and a control. Experimental groups had BSE educational pamphlet alone (BPAM), and a combination of BSE pamphlet and BSE training (BPAME). The control group had a cervical cancer educational pamphlet. Participants were followed-up for eight weeks after the study implementation. Data analysis was done using SPSS version 23.0. Results: Mean age of respondents was 15.64 ± 1.33years. There was a significant difference in the respondents' level of BSE knowledge between baseline and 8 weeks follow-up (BPAM=5.20±2.28 to 21.56±1.85, t =29.44, 29 p<.05; BPAME=7.03±1.13 to 21.73±1.26, t =31.77, p<.05), and a significant difference in the level of BSE practice 29 between baseline and 8 weeks follow-up (BPAM 0.33±1.21 to 4.90± 1.56, t =13.25, p<.05; BPAME= 32.76±4.66 29 to 63.83±2.80, t =18.99, p<.05). Both interventions had same impact on BSE knowledge (Effect size ES=0.97, 29 p=0.000) but BPAME had a higher impact on BSE practice [(BPAM: ES=0.87, p=0.000) and (BPAME: ES=0.92, p=0.000)]. Conclusion: Both interventions had significant influence on the respondents' knowledge and practice of BSE but combined BSE pamphlet and training had more impact on BSE practice.