MgAl 2O 4 ceramic pellets have been investigated as the in situ humidity sensing elements for proton exchange membrane (PEM) fuel cells. The MgAl 2O 4 powder was synthesized through EDTA-citrates method. When the temperature increased from 50 °C to 80 °C, the ohmic resistance (R) at the whole relative humidity (RH) range decreased by about one order of magnitude while the linearity of Log(R) vs. RH remained constant. The MgAl 2O 4 specimens sintered at 1200 °C with ball-milled powder show the highest humidity sensitivity with 3.3 orders of magnitude change from RH 40% to RH 100%. Homogenous meso-pore (around 50 nm) might be the root cause for high sensitivity and the macro-pore (>50 nm) is the cause for decreasing the sensitivity of the sensing material. A precise optimization criterion for pore size has been proposed for this RH range. The MgAl 2O 4-based ceramic pellets show promising performance for the in situ RH sensing for PEM fuel cells.