Designated CBI court Judge Chaudhary Birendra Kumar Rai also ordered that the convicted Rustam Khan to pay a penalty of Rs 40,000.
On Wednesday, the court had convicted Khan of the murder committed 15 years ago, and had reserved the order for punishment for today.
Chandrashekhar was shot dead by unidentified armed assailants in Bihar's Siwan district on March 31, 1997 while he was campaigning against criminalisation of politics.
Investigation is on against former RJD MP Mohammad Shahabuddin for his alleged involvement in the murder case.
The CBI court had on March 2012 sentenced three persons - Dhruv Kumar Jaiswal, Shekh Munna Khan and Iliyas Bari alias Mintu - to life imprisonment and imposed a fine of Rs 40,000 on each after finding them guilty of the murder.
Mian's case was being tried separately by the CBI court.
