I want to write C# code to convert all mathtype euqation object in word to MathML format? I have no knowledge about the convertion?
The MathType SDK has what you need. You can download it from the Design Science website: http://www.dessci.com/en/reference/sdk/default.htm. Note that "The SDK is provided "as is", free of charge, and Design Science does not provide support for the SDK."