This paper focuses on a reliability model of embedded system architecture using AADL (architecture analysis and design language). It performs transformation from AADL reliability model to GSPN (general stochastic Petri net) model and assesses AADL architecture reliability computation model by the means of GSPN theory. To support the reliability analysis and assessment automatically, this paper designs and implements an AADL reliability assessment model tool, ARAM (AADL reliability assessment model tool), with formal methods. It is integrated into OSATE (the open source AADL tool environment) and uses PIPE2 (platform independent Petri net editor 2) to carry out the reliability assessment of GSPN model. Meanwhile, this paper also presents a case study on the reliability analysis and assessment of avionics flight control system to demonstrate the performance of ARAM.