Introduction: Identification of effective factors in reduction of response time in pre-hospital Emergency Medical Service (EMS) can help a system to improve its services. This study aimed to study the effect of using Global Positioning System (GPS) in ambulances on the average of response time. Methods: Among 132 Tehran's EMS stations, 24 ones were randomly selected and data of response time in all missions in 2009 (without GPS) and in 2011 (with GPS) and number of daily calls in each center were gathered. According to the average number of daily calls centers were divided to three categories: (I) centers with ten and less calls, (II) centers with ten to fifteen calls, and (III) centers with more than fifteen calls. Paired t-test was used to analysis the data. Result & Conclusions: Findings show that the average of response time in 2009 for three groups (I-III) was 10.11, 11.25, and 10.92 minutes respectively. The time was 9.86, 10.71, and 11.58 in 2011 that is significantly (p<0.001) less that of it in 2009. Keywords: Pre-hospital, EMS, GPS, Ambulance, Response Time, Tehran