We report a robust constrain on the possible variation of fine-structure constant , \alpha \equiv e ^ { 2 } / \hbar c , obtained using O iii \lambda \lambda 4959,5007 nebular emission lines from QSOs.We find a \Delta \alpha / \alpha = - ( 2.1 \pm 1.6 ) \times 10 ^ { -5 } based on a well selected sample of 2347 QSOs from Sloan Digital Sky Survey Data Release 8 with 0.02 < z < 0.74 .
Our result is consistent with a non-varying \alpha at a level of 2 \times 10 ^ { -5 } over approximately 7 Gyr .
This is the largest sample of extragalactic objects yet used to constrain the variation of \alpha .
While this constraint is not as stringent as those determined using many-multiplet method it is free from various systematic effects .
A factor of \sim 4 improvement in \Delta \alpha / \alpha achieved here compared to the previous study ( ) is just consistent with what is expected based on a factor of 14 times bigger sample used here .
This suggests that errors are mainly dominated by the statistical uncertainty .
We also find the ratio of transition probabilities corresponding to the O iii \lambda 5007 and \lambda 4959 lines to be 2.933 \pm 0.002 , in good agreement with the National Institute of Standards and Technology measurements .