Hi Raval!
Here is a link with information on Khao Yai NP:
http://www.thaiparks123.com/en/nationalpark.cfm?parkid=17
I have been there twice in -88 and -98, and it's well worth a visit if you are a serious naturelover! It's open year around, but expect it to be very wet and slippery, and lots of leaches too during the rainy season! Sudden torrential rainfalls can make the waterfalls and rivers swell with water in no time, both people and elephants has been swept away by the currents! I would recommend you to go in the dry season!The park is famous for its wild elephants and the diminishing numbers of tigers, but you are more likely to see animals like deer or even a quick glimpse of the gibbon high up in the canopy. It's a haven for ornithologists and worldfamous for it's Hornbills. I send you a couple of pictures from -88 with a Greater Hornbill being chased by a Racket-Tailed Drongo! Have a nice trip! TJ
