The behaviour of the core of rockfill dam with slanting clay core was investigated with the Jebba main dam, Nigeria as a case study. The stress-deformation behavior of the dam was studied using the finite element mathematical modeling technique. The actual fill properties were utilized and two loading scenarios-no water and full reservoir operations-were investigated. At full reservoir, the actual seepage line, as recorded over the years was used. The linear total stress analysis using the finite element method of analysis was employed to analyze the dam section. Following the finite element mathematical modeling and computer simulations of the models, deformations and stresses distributions were determined. The results showed that the observed longitudinal cracks at the Jebba main dam crest are as a result of potential cracks in the dam core section. The cracks observed at the crest, from analysis, commenced from the dam’s core upstream face, downstream face and core crest. This may have resulted from probable hydraulic fracturing and tensile stresses developments in these regions. The results also showed likely loss of integrity of core due to cracks. However, total collapse may not be experienced but dam performance may be impaired and alarming visible cracks may not be ruled out over time.