This paper presents a survey about the formal specification languages which are used to specify Real-time systems. Real time systems are critical systems such as nuclear missile system, auto pilot system etc. A minor fault in these systems can result into serious consequences. These consequences may comprise threat to human life. Therefore it is very vital to specify the system correctly. To achieve the correct and unambiguous specification, formal languages are used. Formal languages increase the confidence on the specification and hence the overall system. This paper compares formal languages which are used to specify real-time systems such as VDM++, RTSJ, ASTRAL. A comparison criterion is established to select the best suitable specification languages for Real-Time systems. In the end a small example is mapped in to three selected specification languages which conclude that which language provides the best constructs and tools for Real-Time systems. © 2010 IEEE.