Market segmentation is the foundation on which all other marketing actions can be based. It requires a major commitment by management to customeroriented planning, research, implementation and control. The overall objective of using a market segmentation strategy is to improve company's competitive position and better serve the needs of customers. In this paper, we use the concept of market segmentation in diffusion model for advertising a new product considering the external as well as internal infl uences and study the optimal advertising effectiveness rate in a segmented market. First, we discuss the evolution of sales dynamics in the segmented market under the assumption that the fi rm advertises in each segment independently. Further case of a single advertising channel, which reaches several segments with fi xed spectrum, is also discussed. Such problems have not been discussed before. The optimal control theory is applied to study and solve both classes of problems.