Two suspects were immediately apprehended after a brief exchange of fire with soldiers.
Both were taken to undisclosed place for questioning.
Deputy commander of the First Army Region Maj Gen Apirat Kongsompong rushed to inspect the scene of explosion Friday night, along with explosive ordnance disposal police.
Royal Thai Police commissioner Pol Gen Somyot Phumphanmuang later said the grenade used in the attack at the Criminal Court was the Chinese RGD 5 type grenade which was same as those used during by perpetrators during the anti government rallies last year.
The grenade landed at the car park area of the Criminal Court and caused a 10- centimetre deep and a 20-centimetre wide hole on the floor.
EOD police reinspected the court compound again Sunday morning and discovered a loaded .357 revolver dropped at the scene, and two casings of 11 mm pistol, indicating the perpetrators intended to attack the court compound with more than one type of weapon.
Army spokesman Col Winthai Suwari said intelligence report pointed that there is still attempt to incite unrest in the country.
He said the army chief has been informed of the incident and has ordered security officials to monitor and keep watch on all unusual movement.
Close cooperation has bee established with the police to investigate the explosion last night, he said.
He confirmed two bomb suspects were immediately arrested and were being detained for questioning.