Abstract Background It is unclear whether the number of paramedics in an ambulance improves the outcome of patients with out-of-hospital cardiac arrest (OHCA) or not. Methods and Results This study was a prospective, observational study conducted on patients with OHCA. Patients were divided into the One-paramedic group (Group O) and the Two-or-more-paramedic group (Group T) and we analyzed the differences. Patients who were treated with only basic life support during transportation, and whose cause of cardiac arrest were extrinsic cause such as trauma and poisoning were excluded. Good neurological outcome was defined as cerebral performance category (CPC) 1 or 2. In Group O, there were 1516 patients (male/female, 922/594). In Group T, there were 2932 patients (male/female, 1798/1134). Return of spontaneous circulation (ROSC) was obtained in 528 patients (34.8%) in Group O and 1058 patients (36.1%) in Group T (p = 0.589). 320 patients (21.1%) in Group O and 656 patients (22.4%) in Group T were admitted to hospital after ROSC (p = 0.461). At 90 days, there were 57 survivors (3.8%) in Group O and 114 survivors (3.9%) in Group T (p = 0.873). At 90 days, 14 patients (0.9%) in Group T had a CPC of 1 or 2, while 30 patients (1.0%) in Group T did so (p = 0.87). From the results of logistic regression analysis, age odds ratio (OR): 0.983, 95% confidence interval (CI): 0.952 – 0.993, witnessed OHCA (OR: 4.583, 95% CI: 1.587 – 13.234), and shockable rhythm as first documented (OR: 19.67, 95% CI: 9.181 – 42.13) were associated with good outcome. Conclusion The number of paramedics in an ambulance did not affect the outcome in OHCA patients.