Barbera led much of the 20 lap race, pulling a 2.3secs lead through the middle stages, but the Spaniard began to lose pace thereafter and he was hunted down by pole sitter Alex Debon, Polaris World rider Pasini and KTM's Mika Kallio.
Debon finally dived inside Barbera at the hairpin to take the lead with two laps to go, but Barbera struck straight back. However, as the pair concentrated on avoiding each other, Pasini cut inside them both to steal the lead!
The Italian, known for his extraordinary run of bad luck last season in 125GP, never looked back thereafter to take a memorable win by half a second from Barbera, while Kallio claimed the final podium position.
Debon, Jorge Lorenzo's replacement, dropped to the back of the lead pack.
Early race leader Thomas Luthi was forced to serve a jump start penalty and finished 15th.
Race results - Qatar:
1. Pasini (Aprilia)
2. Barbera (Aprilia) +0.557secs
3. Kallio (KTM) +1.029secs
4. Debon (Aprilia) +1.418secs
5. Takahashi (Honda) +12.944secs
6. Bautista (Aprilia) +14.480secs
7. Abraham (Aprilia) +16.721secs
8. Locatelli (Gilera) +18.987secs
9. Espargaro (Aprilia) +32.232secs
10. Faubel (Aprilia) +41.102secs
Laverty (Aprilia) DNF