The stability of platooning vehicles that consist of connected vehicles with cooperative adaptive cruise control features is crucial for the safe and efficient operation. The stability issue, however, must address the heterogeneity in the highway. The heterogeneity may compromise platooning vehicles' potentials such as traffic efficiency, safety, smoothness, and fuel efficiency. For instance, the existence of human driving vehicles on the highway enables mixed platoon vehicles to occur. Nevertheless, the strict string stability is the most used to judge the platooning vehicles' stability, can be only used when the platoon consists of all connected autonomous vehicles. After that, several criteria come up to solve mixed platoons such as head-to-tail string stability. However, head-to-tail string stability allows the middle platoon's oscillation to compromise platooning vehicles' potential. This paper proposed a new string stability criterion to deal with mixed traffic without compromising the platooning vehicles' potentials. It transforms the platoon into the sub-platoon based on human driving vehicles' position. The mixed platoon stability is checked based on each sub-platoon interconnection, where the following sub-platoon does not magnify the magnitude of the perturbation. Therefore, it is called sub-platoon strict string stability. The effectiveness of sub-platoon strict string stability is shown through the analysis and numerical example.