To design and develop secure operating systems, the BLP (Bell-La Padula) model has been widely adopted. However, user's security label in the most developed systems based on the BLP model is inherited to a process that is actual subject on behalf of the user, regardless whatever the process behavior is. So, there could be information disclosure threat or modification threat by malicious or unreliable processes even though the user is authorized in the system. These problems can be solved by defining the subject as (user, process) ordered pair and by defining the process reliability. This paper presents an extended BLP (E-BLP) model in which processes are grouped into two subdivisions, common and public, and the execution of a process is controlled by DRC (Dynamic Reliability Check) to prevent the malicious behavior of the process. Therefore, this model mitigates the confidentiality threat, integrity threat, and also gives enhanced usability of the system.