About G8

The G8 summit will be held at Huntsville, Ontario, Cananda. It is a meeting for eight countries from all over the world. But the G8 doesn't include China. I don't understand why. Is China not big or strong enough? What is the answer?
I saw the news that HuJintao, the president of China, will attend the G20. I think G20 is a meeting of finance ministers. If I were president of China, I would not do that.