A manual jaw opening device with force acting on molars includes: a manual pivotal rotating device; and a pair of opening arms, respectively formed with a first end and a second end, wherein the first end is connected to the manual pivotal rotating device and the second end is equipped with a molar engaging seat; when the pair of opening arms are disposed in the mouth of a patient and the two molar engaging seats are respectively abutted against plural upper molars and plural lower molars, an operator is able to separate the pair of opening arms through operating the manual pivotal rotating device thereby allowing the mouth to be opened.