Pistorius, 29, could face 15 years in prison after a court last December convicted him of murder.
The ruling came after the state appealed against a manslaughter verdict, for which Pistorius has served nine months of a five-year sentence.
Pistorius shot Reeva Steenkamp four times through a locked toilet door in February 2013.
Pistorius is a six-time Paralympic gold medallist whose legs were amputated below the knee as a baby. He made history by becoming the first amputee sprinter to compete at the Olympics, in 2012 in London, running on prosthetic "blades".
At the court session in Pretoria on Wednesday, Pistorius faces the prospect of many more years behind bars.
Murder carries a 15-year minimum jail term in South Africa. Judge Thokozile Masipa will have to consider whether there are compelling reasons for deviating from the prescribed sentence.
However, the sentence may not bring an end to the drawn-out case as both the prosecution and defence have the right to appeal it.
His unsteady gait sought to remind the judge of his vulnerability, which could be a mitigating factor.However, the athlete's critics have cast doubt on this, our correspondent adds.
They argue that a TV interview with Pistorius broadcast just two weeks ago is an aggravating factor. They claim it showed poor judgement and was an insult to the court and Reeva Steenkamp's grieving family.