I need to connect two boards using UART of AVR. I wanted to know how much can be the maximum length of the cable between two baords if VCC is 3.3V. I dont want to use rs232 here.
I can't answer as to the maximum length, but at 9600 baud I wouldn't expect problems at a metre or two, let alone 10cm. Just ensure there is a good ground wire connecting the two boards.
And that assumes it's permanently wired, and working in a typical lab environment, or your home, rather than in the engine compartment, or in orbit or on a mountain top.
Even in the lab, if you plan to unplug and reconnect it regularly, add that info to the question and someone will advise on suitable ESD protection.