The prosecution had asked for the evaluation after a psychiatrist told the court on Monday that Pistorius, who is on trial for the murder of Reeva Steenkamp on February 14 last year, suffered from an anxiety disorder.
Prosecutor Gerrie Nel has argued he wanted to prevent the 27-year-old sprinter, who faces life in prison if convicted, from later appealing on the basis of a disturbed mental condition.
Defense lawyer Barry Roux had argued against the motion, saying it was premature to have the evaluation before he called other witnesses.
The evaluation is expected to take around 30 days and will prolong the trial which began on March 3.
