Influenced by the extensive focus on sustainable development, the assessment of corporate sustainability has become a developing topic both in academic research and in practice. In order that manufacturing companies conform to their commitments to sustainable development, a reliable measurement framework, which allows them to reach an effective performance improvement, is needed. Based on a review of literature in the context of corporate sustainability, there is still an overall lack of sector-specific and empirical research in many areas, including the sugar industry. Most of the previous research has identified criteria and indicators based on theoretical concepts without seeking empirical evidence for how practical they are. Furthermore, efforts to assess qualitative aspects in a standard way and to logically aggregate various performances concerning corporate sustainability are still limited. Understanding of management's attitudes toward the importance and contribution of each criterion is also a gap in the existing literature. This thesis addresses two main objectives. The first is to develop a framework for the assessment of corporate sustainability based upon empirical research methods. The sugar industry in Thailand is selected as the basis of this study. The second objective is to propose a mathematical model to facilitate this assessment. The Evidential Reasoning (ER) approach is mainly applied for this objective. It is able to deal logically with an aggregation of multiple criteria including both quantitative and qualitative aspects. The main advantage of the ER approach is its ability to handle various kinds of uncertainties and incompleteness which always occur when subjective judgments are involved in the assessment. Different attitudes toward the importance of each sustainability criterion are empirically explored through the direct weight assignment method. The original contribution of this thesis is the provision of an implementable tool for companies to self-assess their sustainability performance and to compare this with competitors. There is discussion of practices regarding how a sugar manufacturer can maintain itself in a sustainable way, which companies, not only in the sugar industry but also in other business sectors, can learn from and apply to themselves. Complexities when applying the ER approach are addressed, and solutions are then suggested. This enhances the practicality of the ER approach to cases where the evaluation standards contain overlap events and are not defined under the unique frame of discernment. The variation in attitude toward the importance of sustainability criteria is also explored. This would support decision-making processes where complex corporate sustainability issues are involved.