In Khao Yai you'll find basic Thai food from restaurants that close very early.
With all services controlled by the national park's government authority, food options in Khao Yai are scant, but if you like Thai food you'll be perfectly satisfied as long as you don't mind an early dinner. The largest eating area is the cafeteria located across the road from the Visitor Centre. Instead of a traditional restaurant set up, the massive high roofed cafeteria is arranged like the big food court cafeteria style places found in Thailand's big cities.
There is no table service, so visitors are required to order directly from the food stands and clear their own dishes after eating. We found the Thai food to be very cheap and of a high standard -- red curry with bamboo shoots, khao ka moo (rice with slow roasted pork and hard boiled eggs) and fried kale were all authentic and flavourful.
Best of all, visitors are able to see the food and make their choice based on what looks appealing. Food stands also sell a range of drinks, fresh fruit, and sweets, but alcohol is now prohibited in all of Thailand's national parks. There is however a coffee stand that sells high quality, strong Thai coffee. While the park's other restaurants close down at 16:00 sharp, the cafeteria at the Visitor Centre -- or at least one or two of its food stands -- may stay open later depending on customer volume.
The other eateries in Khao Yai, located at the campgrounds, Zone 4 lodging, and near the trail to Heo Suwat waterfall, are set up in a more traditional restaurant style with menus. There are only about ten offerings and all are simple Thai dishes like green curry, cashew nut, and fried rice for just 30-40B per item. These restaurants are open from 8:00 to 16:00.
If you miss out, the only option is to grab some instant noodles and snacks from one of the convenience shops, so if possible it's not a bad idea to bring some provisions from outside the park.