If i stay with expired passport in Thailand, what could be a penalty?

Jail.

Where are you from? You can generally renew at your embassy

Your visa would expire, you would not be able to leave and eventually you end up in the monkey house.

Is this April 1st again ?

Deportation and blacklisting

You would probably not have a valid permission to stay anymore, meaning you would be on overstay and would have to pay a fine of 500 baht per day you stay in the country, with a maximum of 20,000 baht. You could be detained till you have payed the fine and have a ticket out of the country.

If i stay with expired passport in Thailand, what could be a penalty?

I hold a UK passport and I renewed mine by post with the Embassy in Bkk. I live in Phuket and did it all by post. If you are from UK too I suggest you go to the Embassy web site where it will tell you how to renew your PP and fees involved. If your not from UK I suggest you go to your own country's embassy web site I'm pretty sure they will have the same info. Best of luck

It looks as though Sebia does not have a Consulate in Thailand

The nearest looks to be in Indonesia. If this is up to date get in touch with them.

Serbia Embassy , Thailand

J.1.Cokroaminoto No.109

Jakarta

Jakarta Pusat

Indonesia

Phone:

+62-21-3143560

+62-21-3143720

+62-21-334157

Fax:

+62-21-3143613

Email:

ambajaka(at)rad.net.id

ambajaka(at)indosat.net.id

The above does look to be your best bet to obtain a new passport.

Some more Serbia Consulates here. http://www.embassiesabroad.com/embassies-of/Serbia#13177