Introduction: Underweight (UW) individuals face lot of problems in increasing muscle size and also increasing body weight and fitness levels. Resistance training (RT) plays a very important role in increasing hypertrophy of the muscles one and all in general. (ACSM, 2009) stated that the resistance exercise for healthy adults provides program design recommendations for muscle hypertrophy [3]. The purpose of this study is to find out the influence of low, Medium and high intensity of (RT) on the muscular hypertrophy and selected health related fitness variables among the underweight males.  Method: A group of (N=40) subjects was selected randomly to participate in this study.  The age of the participants was in the range of 18-24 years, (RT) program was employed for 12 weeks, two days in a week, 45 minutes of training per session. The subjects were segregated into two groups namely Group-A (n=20, experimental group), Group –B (n=20 control group). The (RT) was employed on experimental group. Control group was not given any above mentioned special training program. The tests (pre and post) considered for this study was health related fitness variables are as follows; body composition, muscular strength, muscular endurance, Flexibility and cardio-vascular endurance. And test for muscular hypertrophy i.e. (neck, shoulders, chest, arms,  waist, hips, thigh and calf; To compare the mean differences from pre to post test, mean, standard deviation, and t-test was computed by the help of statistica software.  Results & Discussion: 12 weeks of training protocol had revealed significant performance from pre to posttest among the subjects with regard to the selected fitness variables and presented by percentages i.e. BMI (4.45%), bench press (35.12%), sit-ups test (22.34%), sit & reach test (26.83%), and 12 min run & walk test (26.94%). Furthermore the muscular hypertrophy was enhance among the underweight students i.e. Neck (2.61%), shoulder (2.95%), chest (6.75%), arms (10.43%), waist (8.23%), thighs (11.15%), calves (4.08%) ; and lastly hip circumference by (3.33%) had shown reduction in size from pre to post test.  Conclusion:  It is concluded that the influence of training had shown significant performance among the (UW) students with regard to the selected fitness test i.e. (BMI), bench press, sit-ups test, sit & reach test and 12 min run & walk test.  Furthermore the selected muscular hypertrophy of the (UW) students i.e. Neck, shoulder, chest, arms, waist, thighs, calves circumference had shown increased in the muscular size and hip circumference had shown reduction in size from pre to post test.