if two host to host machines use IP but Y MAC address is need in the that communication process.

IP addresses work on the routing layer (otherwise known as the network layer). This layer is responsible for getting information from one network to another network, not necessarily individual computers. Once IP gets the information to the right network, it passes the data off to the data link layer. This layer is responsible for individual hosts within the network and uses MAC addresses to identify them. So IP addresses work between and among networks, MAC addresses work within them.

The reason why IP addresses aren't used within the network is because they really serve a different role than this. They aren't guarenteed to be unique or static. MAC addresses are (almost) 100% unique and (almost) never change, so once a router or switch in a network has info on a MAC address, it generally doesn't change.

I'm not sure I've explained that well, so if anyone can explain it better chime in.