This study investigated the impacts of Jigsaw II cooperative learning strategy on students? academic performance and retention in mensuration concepts among senior secondary school students in Kano state, Nigeria. In this study, four (4) research questions were raised and answered. Also, four (4) hypotheses were formulated and tested at 0.05 level of significance. The study adopted the quasi-experimental non-randomized pre-test-post-test control group design. The target population of the study was all SSI students of public secondary schools in Kano state, totaling 69,736 from which four (4) intact classes from four schools were selected using simple random sampling technique. Two intact classes with 82 students were randomly assigned to experimental group and the other two with 77 students assigned to control group. Thus, the sample size was 159 SS1 students. The study made use of two (2) instruments, Geometry Achievement Test (GAT) and Geometry Retention Test (GRT). GAT was constructed by the researcher and contained 30 multiple choice items. GRT also contained 30 multiple choice items, constructed by swapping the items in GAT. Using split-half method, the reliability coefficient of GAT was 0.74. GAT was used as pretest and as posttest while GRT served as Post-posttest (Retention test). Both experimental and control groups took GAT as a pre-test before the experiment started. Experimental group was then taught mensuration concepts for six (6) weeks using jigsaw II cooperative learning strategy while the control group was taught the same topics for the same period using conventional lecture method, after which GAT was taken by both groups. GRT was administered on both groups two weeks after taking GAT. Data collected was analyzed using independent t-test.. Findings showed that the mean performance score of the experimental group was significantly higher than that of the control group. In terms of gender, findings revealed that there was no significant difference between the mean performance score of males and females when taught by using jigsaw II co-operative learning strategy. It is recommended that mathematics teachers should be trained by State Ministry of Education on how to implement jigsaw II cooperative learning in their classrooms by organizing seminars and workshops in order to improve students? performance and retention ability in mathematics