Background and Aim: Development of phlebitis is a painful and common complication in the application of peripheral intravenous catheter (PIC). This is a prospective observational study performed to identify development rate of phlebitis in application of PIC and the factors that affect the development of phlebitis. Materials and Methods: The study universe comprises of catheters applied on inpatients in the internal diseases clinic of a state hospital, and the sample comprises of catheters eligible to be included in the study. Five hundred and thirty- two PICs applied on a total of 317 patients were reviewed. The patient identification form, information form for peripheral venous catheter and treatment, and visual infusion phlebitis (VIP) assessment scale were used to collect data. Results: 31.8% had phlebitis and a large number of them (79.2%) were Level I phlebitis. There was a significant relationship between having a chronic disease, duration of catheterization and type of fluid used and the development rate of phlebitis. Conclusion: The phlebitis in individuals receiving intravenous (IV) treatment was higher than the rate defined by both the centers for disease control and prevention and IV nurses society. It may be recommended to assess phlebitis by VIP assessment scale and to take preventive measures specifically for development of phlebitis.