The discovery was made just after 8 p.m. near the intersection of Bustleton and Unruh avenues in Northeast Philadelphia.
Police are calling the deaths suspicious partly because officers found the men positioned in the floorboard and middle console area of the truck.
"Where their bodies are located is very unusual. They're not sitting in the driver's seat or passenger seat. They were actually stuffed in the floorboard area in front of the seats," said Philadelphia Police Chief Inspector Scott Small. "So their bodies are positioned...very unusual positions for the two of them, and they are both large males. It's unusual the way we found the bodies."
According to investigators, there were no obvious signs of trauma or a struggle. They say there was no blood or signs of drug use, however, several cans of beer were found in and around the truck.
Investigators say one of the men had an ID on him. He was 64 years old.
Police are still working to identify the other victim; they say he appears to be in his 50s.
Police also say the men are known to be homeless and live in the back of the box truck where there are couches and rugs.
An autopsy of the bodies will determine the cause and manner of death.
Anyone with any information is asked to call police at 215-686-TIPS.