Interstitial La 0.7 Pr 0.3 Fe 11.5 Si 1.5 C 0.2 H x hydrides with large magnetocaloric effect are prepared. It is found that the addition of carbon atoms can lower the rate of hydrogen absorption/release, which is beneficial to an accurate control of hydrogen content. The Curie temperature T C can be adjusted to the room temperature by changing the carbon/hydrogen content. The introduction of carbon atoms leads to a remarkable reduction of thermal/magnetic hysteresis because the first-order magnetic transition is weakened. The maximal magnetic entropy change of La 0.7 Pr 0.3 Fe 11.5 Si 1.5 C 0.2 H 1.2 is found to be 22.1J/kgK at T C =321K without hysteresis loss for a field change of 0–5T.