The electronic Grüneisen constant γe of Al is measured at room temperature using our homemade ultrafast electron diffraction (UED) equipment. The electronic contribution to the thermal expansion of solid is differentiated from the lattice contribution in the time domain when an Al film is heated rapidly by ultrashort laser pulse. By directly probing the associated thermal expansion in real time at room temperature, we differentiate the two in the time domain, and finally obtain the electronic Grüneisen constant γe. With this new method we overcome the restriction of the traditional low temperature methods in measuring the electronic Grüneisen constant γe of nonmagnetic metals.