Hello! 

 

My visa will expire on 9 december(today) and I’ll take a train to Malaysia border. But when I’ll arrive it will be 10 december around 10 am. 

 

That’s mean I overstay 1 day. This is my first time overstay within 3 years in Thailand.

 

What should I do? It will be problem? Immigration ok if overstay 9-10 hours?

You will be fined 500 baht for one day of overstay and be given a overstay stamp. Nothing more than that will be done.

Pay the fine, 500 Baht (no free day at land border) get the overstay stamp, no real issue.

 

Try to avoid doing it again 

 

Thanks!

I'm not sure when/if you planned to return to Thailand, but be aware that Malaysian border-points have been known to refuse-entry to those with previous overstays, even if they have a valid Tourist Visa.  Hopefully, one "less than one day" overstay would not set them off.  In any case, both leaving and if returning, be contrite about the subject - "Mistake, very sorry" vs "Aw, it's no big deal."