They told the Supreme Court of Appeal the six-year sentence was "shockingly light" and he should get 15 years.
Defence lawyers say the sentence handed down by a lower court is appropriate.
Pistorius claimed he shot dead Ms Steenkamp on Valentine's Day in 2013 after mistaking her for a burglar at his home in the capital Pretoria.
The lower court justified deviating from the prescribed 15-year sentence by saying mitigating circumstances such as rehabilitation and remorse outweighed aggravating factors such as his failure to fire a warning shot.
