Gear systems are used to transmit power in the industry when accuracy and synchrony are needed and helical gear systems are used in more accurate and high-speed industries. It is important to ensure that these systems work faultlessly, therefore the detection of the crack location and situation is very efficient in the gear systems. In this research, a new approach is proposed to detect the multi crack location and length in the helical gear teeth. To this end, after giving an explanation of helical gear mesh stiffness and demonstrating the helical gear pair dynamic modeling, the transmission error ratio method is used to detect the cracks locations and lengths. Then, according to solved examples, when the cracks locations are far enough that their effects on the transmission error are completely separated, the cracked teeth and the lengths of cracks can be detected exactly, and when the cracks are in adjacent teeth, according to the cracks lengths and depths and their effects overlap, the number of cracks and their lengths can be detected exactly, approximately or absolutely undetectable.