How to find Router Serial interface MAC address?

We can see Mac addresses on router for the FastEthernet ports via the following command:

i.e. Router # Show IP ARP

Serial interfaces don't use MAC addresses because MACaddresses are only used with Ethernet

Serial interfaces don't use MAC addresses because MACaddresses are only used with Ethernet. Serial uses PPP,HDLC, Frame Relay, etc for its layer 2, none of which useMAC addresses. 

Router Serial int dont has MAC address. only Ethernet &Fast-Ethernet has MAC. hit sh int f0/0....etc...... It willshow MAC ID.

It is highly difficult to know mac addresses using serial interfaces ...

There is no MAC used in the WAN network.

As MAC works on layer 2 the same way you need to configure the encapsualtion which would work on layer 2.

Basically MAC Address are related to Switches used in LAN... MAC addresses are not used in WAN, so Routers don't use MAC addresses instead they use IP to recognize... 2ndly Serial interfaces are not used much nowadays, they became obsolete instead now ISPs provides termination on Ethernet Link...

Mac-addresses are relevant on Ethernet interfaces. Serial interfaces are not Ethernet interfaces so they don't have one.

where serial interface not have mac addresses .it is have on ethernet

There's no such a thing in serial interfaces. MAC address exists within a NIC cards.

The Question is not correct. MAC or Physical address is entirely related with Ethernet port.