I have stamp in my passport allowing me to stay till 18'th of August and I plan to take a flight out that date in the morning. Will it consider an overstay or not? I am not worry about money,just dont want to go through the hustle and i have multi non B visa, so can it affect it?

It means you have to leave on the 18th, so it is no overstay.

AFAIK you should leave the country on the date that is stamped in your passport: "admitted until xx-xx-xxxx"

Usually, the date of entry and the date of departure are included in the period of permission of stay.

Beware: Some airliners depart at 3 a.m. in the morning. You are required to check in approx. 3 hours prior to the departure.

Some Immigration officers will see the day of actual departure as your last day in Thailand, although you pass immigration and customs a day prior to that.

Play it safe.