Background: Mobile phones are increasingly becoming part of the daily life of today’s youth. It has become an important accessory because of the various facilities they offer especially the internet. The Covid -19 outbreak and the subsequent lockdown has put all the people inside the home. To keep themselves engaged, most of the people are clinging onto their smart phones. All the academic sessions are taken through online. Hence the aim of the present study is to assess the mobile phone usage in undergraduate medical and dental students during Covid 19 lockdown. Methodology: A cross sectional study was conducted among 200 undergraduate medical and dental students of ACS Medical College and Thai Moogambigai Dental College, Chennai. Data were collected by pre-tested questionnaire in Google form and it was analysed using MS Excel software. Results: Out of 200 students participated in the study, 112 were females and 88 were males. Only about 44 students (22%) used 90-100 % of internet data before lock down and now about 102 (51%) students used 90-100% of internet data pack during lock down. About 21.5% of students spent more than 10 hours in their mobile phone duringthis lock down period. Majority of the students (56.5%) used mobile phones for entertainment purpose more than academic purpose (45.5%). There are also many health problems found to be reported due to usage of mobile phones like headache, sleep disturbances, fatigue, dry eyes etc. Conclusion: The results of this study confirms that there is excessive use of mobile phone among medical and dental students during lock down period which give an alarming indication that the students should not get addicted and more dependent on mobile phones that can lead to mental and health problems in them after the release of lock down. Parenteral control, students self discipline and conduction of online classes for a limited time can help them to overcome this.