The electronic structure of the excited states and triplet formation dynamics in the acridine microcrystal have been studied by femtosecond diffuse reflectance absorption spectroscopy and picosecond fluorescence spectroscopy. It was confirmed that an excimer was formed in the microcrystal within a few picoseconds after excitation. The time constant of triplet formation from the lowest excited singlet (S1) state was obtained to be about 900 ps at room temperature, which is about ten times slower than that in an aprotic solvent. Also, triplet formation was not observed up to 6 ns at 77 K. Slow intersystem crossing in the crystal was explained in terms of a selective stabilization of the 1ππ* electronic configuration due to excimer formation. An important role of intermolecular interactions on the intersystem crossing process of the crystal is discussed.