A triple-CNG (Compressed Natural Gas) cylinder mounting device for a vehicle for mounting a single-gas cylinder placement unit (2) and a dual-gas cylinder placement unit (3) onto the vehicle frame (1) having a right frame member (11) and left frame member (12) partitioned by a front cross-member (13) and a rear cross member (14). The single-gas cylinder placement unit (2) is disposed at the front part of the vehicle frame (1). The single-gas cylinder placement unit (2) is locked by the locking means on the right frame member (11) and/or the left frame member (12) and/or at the front cross-member (13). While, the dual-gas cylinder placement unit (3) is disposed between the middle and rear parts of the vehicle frame (1). The dual-gas cylinder placement unit (3) is locked by the locking means on the frames (11) and/or (12) and optionally on the rear cross-member (14).