Carrier lifetimes in 4H-SiC epilayers are investigated by differential microwave photoconductivity decay measurements. When the Z1/2 concentration is higher than 1013cm-3, the Z1/2 center works as a recombination center. In this case, carrier lifetimes show positive dependence on the injection level (number of irradiated photons). On the other hand, other recombination processes such as surface recombination limit the lifetime when the Z1/2 concentration is lower than 1013cm-3. In this case, carrier lifetimes have decreased by increasing the injection level. By controlling the Z1/2 concentration by low-energy electron irradiation, the lifetime control has been achieved.