'The calculus of Mobile Ambients derives its process primitives from the pi-calculus. It introduces the notion of a bounded environment(the ambient) where processes or mobile agents co-operate. An ambient consists of a set of local agents and possibly other subambients. Ambients are moved as a whole under the control of the enclosed agents, which are confined to their ambients. This report presents a variant of Mobile Ambients. It addresses the security issues of robustness against malicious tampering, access control and execution safety by introducing the concept of digital signature and blocking into ambients. The model of Digitally Signed Ambients with Blocking on Capabilities(DSABC) is similar to the Java Sandbox model for JDK2.0 and Umbrella project developed for Linux security. A type system which captures various security policies has been presented based on the proposed calculus. '