Few days ago I remember that my DTAC dprompt prepaid credit was still 300 something Baht. Then I sent 3-4 SMS to abroad (which I think costs not more than 9-10 Baht per SMS). After then, I never made a call, accessing GPRS, or anything.

Later the next day, suddenly I couldn't make a call or send SMS at all, and when I checked *101# my balance became -10 Baht. Yes, it's negative.

I activated GPRS at DTAC shop when I purchased the card last month. Is there a monthly fee? If not, where did my credit go? Why it can be negative?

dtac doesn't charge a monthly fee, but it does charge a fee of 1 Baht/minute.

My guess would be that somehow your phone maintained a gprs connection, and thus your credit got eaten away.

Some phones do keep the gprs connection, for example for messaging services. This is mainly used with providers charging per data traffic as opposed to time based billing.

I know in my Nokia phone I had to change a setting to only use gpres when needed!