Mobile systems interact with many autonomous entities and multiple services that provide a ubiquitous environment for societies. In this environment, trust to security is a challenging issue. Entities and services have unpredictable behaviors therefore conventional trust models have limited accuracy for security related computations due to static nature of the models. Adaptive trust computations are needed to make accurate trust decisions. In this paper, we propose an adaptive approach to compute trust for mobile security. We evaluate the approach with different scenarios and comparisons to show the adaptive property.