Does anybody know if a USB modem will send my computer MAC address to the internet service provider when I using it?
I mean If two computers use the same USB modem will the internet service provider know that two computers are sharing that modem?
Your computer doesn't have a MAC address on its own, a MAC address is something related to the network adapter. So your USB Modem (I'm assuming you're talking about those 3G/4G USB Modems) has its own IMEI (or MAC address in some cases), and when you use it to connect to the Internet your ISP will see it.
If your ISP doesn't require you to install an application† to use for the connection, then I don't see how they can find out that you're using the same USB Modem on two different computers.
† Such application might gather some data about your computer and come up with a sort-of identifying signature comprised of information about OS installation and/or hardware.