I am using RB750 for load balancing for 3 WAN port and 2 LAN port which is serving about 150 clients and 50% of this client are up all time. My router send this message in log " router was rebooted without proper shutdown by watchdog timer". sometimes it takes 100% cpu usage and i can not login. What is the problem?

You need to post WAY more information to get an answer that isn't a vague guess.

http://wiki.mikrotik.com/wiki/Manual:System/Watchdog

Did you configure an IP address to watch? If yes, that address became unreachable. Investigate why. If no, there was a kernel panic or system lockup. This could be caused by over utilization. The RB750 is a very cheap SOHO board for small amounts of traffic. What kind of traffic patterns are you seeing? What version are you running? What services is the router performing for the network? If you use version 5.x, you can use the profiler to see what router tasks are taking how much CPU resources. What tasks are running high?