The International Society of Automation has recently released ISA100.11a as a standard for reliable and security application in industrial application, in addition to be compatible with existing wired and wireless networks. ISA100.11a, like WirelessHART, uses TDMA scheme in medium access layer to provide deterministic services. However ISA100.11a adopts the CSMA-CA mechanism with priorities for, e.g., retransmission from failure on dedicated links, sporadic data, network configuration. This paper evaluates ISA100.11a CSMA-CA by simulation, considering effects of backoff procedure and priority setting to probability of collision and successful using slot. It is shown higher number of priority classes brings better network utilization and less packet goes over their lifetime, although pay for high accuracy time synchronization. Moreover, extensive simulations in this paper can be used as a basic step to understand this new promising standard.