Due to the rapid changes of business environment and changing technologies, adaptability and measurement of the degree to which software is adaptable are critical factors for survival of software systems. Software are often claimed to be adaptable, however there is no explicit and concrete definition of architecture adaptability. This paper first gives a definition of architecture adaptability, and then presents a metric and a quantitative evaluation approach to evaluate the architecture adaptability based on adaptability scenario profile and impact analysis. The purpose of this study is to provide a structural adaptability evaluation method to support architecture improving and decision making for choosing among candidate architectures.