She will begin reading her decision Thursday.
The judge has to answer several questions to reach her verdict on the murder charge, one of four charges Pistorius faces.
First -- and most important -- did Pistorius know Steenkamp was behind the door in his bathroom when he fired four shots through it on Valentine's Day last year, killing her?
If the answer is yes, beyond a reasonable doubt, Pistorius knew he was shooting at his girlfriend, then the judge will find him guilty of murder.
Later, she'll have to decide whether it was premeditated. If she thinks it was, he could be sentenced to life in prison. In South Africa that means he'll have to serve at least 25 years.
If she thinks the murder wasn't premeditated, Pistorius could get a minimum of 15 years behind bars.
But if Masipa is not convinced that Pistorius knew Steenkamp was behind the door -- if there is any reasonable doubt of that -- he will not be convicted of murder.
Pistorius argues he made a mistake -- that he thought was trying to defend himself from an intruder. If Masipa accepts that as plausible, she will then have to decide if his mistake and his actions were reasonable.
If she says they were not, she'll find him guilty of a crime called culpable homicide. There is no minimum sentence for culpable homicide in South African law -- Masipa will decide the punishment herself, taking South African precedent into account.
And there's one final possibility.
If the judge does not think the prosecution has proved its case -- if she believes there is a reasonable chance Pistorius made a reasonable mistake and responded reasonably -- she will find him not guilty, and Oscar Pistorius could be a free man.
Masipa will make her decision with the assistance of two experts known as assessors. South Africa does not have jury trials.
Pistorius faces three other weapons charges in addition to the murder accusation.
Two charges are related to allegations that he recklessly fired a gun in public -- once in a restaurant in 2012, and once out of the sunroof of a car in 2013. Pistorius denies both.
The maximum penalty for each charge is five years behind bars. If he is convicted of either, he could face a lesser sentence, such as a fine or the loss of his gun license.
The final charge is that Pistorius was found to have ammunition without a proper license when police searched his home after he killed Steenkamp.
Pistorius says he was storing the ammunition in his safe for his father.
The charge is potentially a very serious one. If he is found guilty, he could face up to 15 years in prison, though Masipa could opt for a lesser punishment such as a fine or the loss of his gun license.