How should I do to make my small application run in background mode?Please reply to me.Thanks.

You really need to provide more details here. You need to explain what the application does, why you think you need to have it run in the "background", What you expect it to be doing wehile in the background, what platform you are programming for, what language you are programming in and anything else you think might be relevant.

I'm not a coder so I'm not sure I can help you with any actual coding. But I can help you provide the info needed so a coder can answer.