The visitors have to win to stand any chance of reaching the final whereas a victory for the Ospreys, who have won one of their two games so far, will edge them a step closer.
In the other game, the Dragons travel to Narberth to take on the unbeaten Scarlets.
Two wins from as many games has left last season's winners five points clear of the Ospreys after two rounds of this season's competition. A third victory could leave them as much eight points clear.
The Scarlets will have to do without the likes of Darren Daniel and Liam Davies, but with home advantage, they should be capableof inflicting a second successive defeat on the Dragons, who lie in third place with a 50 per cent record from their opening games.
Round One results
Scarlets 27 Ospreys 18
Dragons 18 Blues 13
Round 2 Results
Blues 26 Scarlets 30
Ospreys 22 Dragons 13
U20 Week 3 Fixtures
Ospreys v Blues, at St Helens, Kick Off 7.15pm
Scarlets v Dragons at Narbeth, Kick Off 7.15pm
Cardiff Blues Under 20's Team selected for 30th November 2005 v Ospreys (Kick off 19:15)
Jamie Roberts, Kristian Baller, Nick Gill, Adam Thomas, Lewis Williams, Gareth Williams Davies, Lee Scarfe, Adam Powell, James Cullen, Matthew Larsen, James Down, Sam Whadan, Alex Murphy, Ross Johnston (c), Rhys Gill
Rory Woods, Jack Bird, Liam Thomas, Aaron Fowler, Kieron Jenkins, Nick Eaves, Daniel Thorne / James Bull