The main purpose of this study was to assess how well model secondary schools inrnimplementing school improvement program (SIP) in Oromia Regional State have developedrn(built) capacity for sustainable improvement. To this effect, the study was employed concurrentrnmixed method design. Five level Likert Scale questionnaires were used to collect quantitativerndata from teachers, school principals, department heads, and students. On the other hand,rnqualitative data collection methods were interview, document analysis, student focus grouprndiscussion (FGD), and observation. The informants for interview were teachers, schoolrnprincipals (the heads), supervisors, and Parent Teacher Associations (PTAs). Mean andrnstandard deviations were the major descriptive statistics for analyzing the quantitative data.rnAmong inferential statistics, independent sample t test, Mann-Whitney U test, and one-wayrnANOVA were applied at ï¡ = 0.05 significance level. For the analysis of qualitative data,rninductive process of data reduction and categorization into themes were applied apart fromrndescriptive narration. It was found that the schools have good capacity in most of the aspects ofrncapacity dimensions included in the study in spite of certain gaps. Planning capacity, leadershiprncapacity, staff performance with a sense of professional learning community (PLC), thernempowerment and the engagement of students, and the engagement of parents/community werernfound to be adequate. The major capacity gaps include the learning norm/practice of the staffrnwas not as adequate as collaborative norms/practices; weakness in the quality of school selfevaluationrnand in consideration of the feasibility of school improvement objectives/targets; andrnlack of student self-control of their learning. Besides such capacity gaps, the study was foundrnseveral challenges that have adverse effect on the sustainability of school improvement. Fromrnthe finding, it is plausible to conclude that the schools have good capacity to sustain thernimprovement they embarked on. However, it requires that the capacity gaps and challengesrnidentified in this study should be resolved or ameliorated. Including the schools, each party atrndifferent levels of education management (woreda/district, regional education bureau andrnministry of education) has important part to play in resolving and/or ameliorating the capacityrngaps and the challenges. Among others, the regional state should give attention to building thernleadership capacity of the schools by attracting and selecting competent and experiencedrnteachers to school leadership position. In the schools, although the collaborative normsrnpractices were remarkably appreciable, attention should be given to improve and deepen thernlearning of the staff and the students. Besides, capacity limitation in terms of financial andrnphysical facilities needs to be resolved. Government block grant to schools should grow as withrnthe school grant and the community contribution to solve the problem in the inadequacy ofschool facilities and infrastructures