The former Olympic and Paralympic runner is charged with premeditated murder in the shooting death of Steenkamp, who died after Pistorius fired shots through a locked toilet door in a bathroom in his home.
Pistorius claims he fired in self-defence, believing his girlfriend was an intruder. The prosecution has argued Pistorius shot Steenkamp after an argument.
Pistorius's testimony was cut short Tuesday after the judge called for an early adjournment.
Pistorius started to cry loudly while testifying about the moments he first realized he shot Steenkamp on Valentine's Day last year.
"I sat over Reeva and I cried," he said.
Judge Thokozile Masipa called for a brief break and then adjourned the session for the day upon her return to the courtroom.
The defence started presenting its case on Monday. Pistorius is their second witness, and more than a dozen witnesses are expected to testify for the defence.
During the first four weeks of the trial, the prosecution presented its case, which included sometimes graphic testimony from experts and neighbours.
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If convicted on the charge of premeditated murder, Pistorius faces a minimum of 25 years in prison before the chance of parole.
The judge alone will determine his fate, as South Africa does not have trial by jury.
Timeline: Oscar Pistorius