I have wrapped log code in main.cs to catch the exception, but the app build by monotouch usually crash in iPad, and I cannot found any log. (some code contain multithread opera
Native crashes can't be turned into managed exceptions (the state of the process is invalid, so there is no way to continue safely).
There are two types of information that will help you (or help others help you) figure out what's going on:
You can find the crash report in Xcode's Organizer, and the device log in MonoDevelop's iOS Device Log pad. Here is a more detailed description how to find each.