Define 'late night'.
There's not a whole lot around there, Tir Na Nog and Table 31 are pretty close, Tir Na Nog is mainly a bar so it's open late, I've had decent food there in the past but haven't been in quite a while. Table 31 has a bar and the food there is good (a little pricey for what it is) but I'm not sure when the kitchen closes. If you don't mind walking a little further, there's Tinto at 20th and Sansom. The food is great, you can sit at the bar (if you can find a seat) and the kitchen is open till midnight on Friday, plus there's a Capogiro across the street for gelato. The Oyster House on the 1500 block of Sansom is open till 11, and the Good Dog Bar on 15th south of Walnut is open late, I'm not sure when the kitchen closes but they make one of the best burgers in the city.
Good Dog (15th btwn Locust & Spriuce) serves very good bar food (in the running for best burger in town) until 1am. This is probably your best bet.
Pub & Kitchen is a bit further away at 20th & Lombard and also serves until 1am. People love the food here but I've had mixed experiences. In any case, the drinking scene gets very crowded on the weekend and will be pretty hard to get a spot at the bar where you can eat in peace; maybe you can get a table if you're lucky.
For more casual, there's a late night pizza place on 20th above Walnut. Open til at least 2am on the weekend. Just up the street at Sansom there is Capogiro, a gelateria, which is open until 11:30 (maybe later on the weekend, I'm not sure). Their gelato is awesome if you want to make ice cream a dinner ; they actually make good sandwiches too, but I don't know if they serve them that late. You can get a gelato con brioche (gelato sandwich served on a brioche roll), though.
For very late, there is Little Pete's diner on 17th between Walnut and Locust. It's 24 hours. Food is passable but I'd consider it a last resort.
117 S 20th St, Philadelphia, PA 19103
Good Dog Bar & Restaurant
224 S 15th St, Philadelphia, PA 19102
Pub & Kitchen
1946 Lombard St, Philadelphia, PA
132 S 20th St, Philadelphia, PA 19103
219 S 17th St, Philadelphia, PA