The crash happened around 2:05 p.m. Thursday in the area of Route 413 and Route 332 in Newtown.
Authorities say that a delivery truck was pulled onto the shoulder of the roadway. Witnesses tell police that a school bus moved onto the shoulder and hit the truck.
The bus driver indentified as - 75-year-old Janet Callan - was airlifted to Temple University Hospital in critical condition.
68-year-old Eleanor Weaver - a passenger on the bus - was taken to Saint Mary's Medical Center where she was treated and released.
Police say the 43-year-old truck driver and his 29-year-old passenger - both from Philadelphia - were inside the truck at the time but were uninjured.
No children were on the bus at the time of the crash.
Investigators say there were no mechanical issues on the truck that contributed to the accident.
The inspection of the bus has not yet been completed.