Kathleen Peace took to the witness box at Birmingham Crown Court where her son Philip Peace, 42, of Himley Road, Dudley, is standing trial accused of murdering his five-month-old daughter Summer.
Prosecutors allege Summer suffered fatal brain injuries after being intentionally shaken by Peace, which he has strongly denied.
Answering questions from her son's defence council, Michael Turner QC, Kathleen said: "Philip was a brilliant dad. He never raised his voice or told the children off. He was a gentleman."
She described him as being a "very good" child as a youngster who was "quiet and loving".
Coverage of the court case:
Asked how she first reacted upon hearing her son had been accused of murdering Summer, she said: "Disbelief."
Summer died on September 9, 2017, after suffering a head injury. She also sustained 11 rib fractures.
The baby girl collapsed around 25 hours before her death where her father administered chest compressions and mouth-to-mouth resuscitation, the court heard.
The defence for Peace is currently taking place.
On Monday, the defence opened its case with Peace answering questions from Mr Turner, where he denied ever shaking or intending to cause Summer serious injury.
He told jurors: "No of course I would not [do that].
He has denied the murder and manslaughter of Summer.
The trial continues.