A zoom mechanism of imaging lens module is disclosed which includes a first lens carrier defining an optical axis, a second lens carrier, a spring connecting with the two lens carriers, a guiding means, a rotary cylinder for holding the two lens carriers and a stationary cylinder for holding the rotary cylinder. The rotary cylinder defines two cam rims, each cam rim has a stem, a beginning rim, an end rim, a guiding rim connecting with the beginning rim and the end rim, wherein the beginning rim and the end rim are formed perpendicular to the optical axis and arranged to space a certain distance therebetween in circumferential direction and axial direction respectively. The spring and the rotary cylinder push the two cam rims to move along the guiding means. The stationary cylinder defines a leading slot which extends in circumferential direction and radial direction for the stem passing through.