can i book hotel from bangkok airport and pay in cash? i don't have a credit card?

Why do u want to make a booking in the Bangkok airport? Just visit the hotel directly and take the room. Or make a call to the hotel and reserve a room. Advice them you are going to pay by cash on check-in.

Yeh, call the hotel and make the booking. Once you get there tell them you don't have the credit card and if they ask put in the cash deposit.
Hotel always respect credit card around the world. I once pay for hotel in Singapore in cash and that raised their suspicion whether I was a drug trafficker.

Actually you can get a very good rate from many hotels at the airport hotel and tour desks, many times a much better rate than the walk-in rate and I'm almost certain they will accept cash for payment. As Ice Cube said, most hotels require a credit card for deposit, but many will accept cash, so have some cash on hand. You might also want to get a debit card if you don't already have one, most hotels will accept debit cards for deposit.

Can you book a hotel at the airport using cash - yes. But, some of the hotels on their "select list of hotels" are not always in the more popular areas of Bangkok. Try for a hotel near the Skytrain.

Cheaper hotels in the 800 baht to 1300 baht range often take cash. I have never used a credit card for any hotel in Thailand in the past 11 years. But, I only stay in cheap places (with air, hot water, private toilet, cable TV,) paying cash has never a problem for me.

If you arrive at the airport real late at night you might want to book ONE night and then check out the area and see what you can do for a better location or cheaper place.

You don't mention any price range, how much do you want to spend per night! In the Sukhumvit area (near the Nana Skytrain) there are lots of hotels in all price ranges. Khao San Road is a popular destination for younger travelers.

If you arrive during the day or early evening hours I would go into town and find a place. The suggestion that you do some telephone calls from the airport is good advice, you can often reserve a room with your name (cheaper places) and tell them you are on the way. Make sure you have a good address for the taxi driver to use to find the hotel.

The following message boards have some specific hotel recommendations.

Virtual Tourist
http://forum.virtualtourist.com/

Trip Advisor
http://www.tripadvisor.com/ListForums-g2-i510-Asia.html

Lonely Planet Thorn Tree Forum
http://www.lonelyplanet.com/thorntree/index.jspa

Good luck.