Superconductivity of polycrystalline Sr and Co codoped Pr1-ySryFe1-xCoxAsO samples was investigated by measuring resistivity, dc magnetic susceptibility, thermopower, and Hall effect. In the PrFe1-xCoxAsO samples with only Co doping, Tc reaches a maximum of 16 K at x=0.075-0.1, and the variation of Tc with Co content (x) is a dome-shaped curve. In the Co and Sr codoped Pr0.8Sr0.2Fe1-xCoxAsO system, the phase diagram separates into two regions. The thermopower and Hall data indicate that the hole-type charge carrier dominates in the low Co doping region (x ≤ 0.075) and the system becomes electron type in the high Co doping region (x ≥ 0.075). Tc first decreases with increasing Co content to a lowest Tc of 3.5 K at x=0.075, then it increases and reaches a maximum of ~16 K at x=0.15. The transition from hole-type to electron-type superconductivity in the nominal Pr0.8Sr0.2Fe1-xCoxAsO system around x=0.075 implies that the charge-carrier density is one of the decisive factors to control superconductivity. [ABSTRACT FROM AUTHOR]