first i used 123.456.789.74/29 ip for my mikrotik router. I used for lan 2 subnets thats are 192.168.0.0/24 and 10.0.3.0/24. when i wanted to use public ip for my customer then my isp provide 123.456.789.80/29 ip block and i used it in my LAN interface. that time i used it in one LAN interface. But now it is not working without src-nat. Though it work with src-nat but web page become too slow. I use dst-nat to redirect to my proxy for private ip.

WAN ip = 123.456.789.74/29 (public ip)
LAN ip = 192.168.0.0/24(private ip), 10.0.3.0/24(private ip), 123.456.789.80/29(public ip)

How can i solve it? please help me

You should not use IP addresses from the same subnet on different interfaces.

I can offer you two solutions.

1) Use currently available address space and setup NAT,
http://wiki.mikrotik.com/wiki/How_to_li ... Local_ones
Setup NAT clients for the clients, which need public IP address access.
Use masquerade for regular clients.

2) Ask your ISP for another public IP address network, which is routed under your router IP address.