Nomophobia is rising among high school students, especially in light of the Covid-19 pandemic, such as social distancing, long-term online learning, and lack of social support. Many studies have also shown that students with high levels of nomophobia have a higher risk of stress. However, very few researchers are interested in studying loneliness as a mediation factor for the relationship between nomophobia and stress. This study investigated whether the loneliness factor is a mediator in the relationship between nomophobia and stress in Vietnamese high school students. Participants include 556 Vietnamese high school students. Participants completed the Nomophobia Scale, Depression Anxiety Stress Scales 21, The UCLA loneliness scale version 3. In this research, to test variable relationships, the mediation analyzing method by the PROCESS macro 3.5 will be applied. The research concludes that for Vietnam high school students, the mediating role of loneliness was identified in the relationship between nomophobia and stress. It is a suggestion through this research that nomophobia preventive and mitigating measures should reduce loneliness in students.