PURPOSE: To provide a radical lithium phthalocyamine crystal that can be used for measurement of molecular oxygen in vivo by electron paramagnetic resonance(EPR).

CONSTITUTION: The radical lithium phthalocyanine crystal of formula II having a crystal structure of tetragonal system, which belongs to space group P4/mcc, is obtained by dissolving dilithio-lithium phthalocyanine of the formula I (R1-R16 are each deuterium or hydrogen) into acetonitrile solution containing ≤50ppm water, and then oxidizing monoelectronically by constant current in the presence of supporting electrolyte formed out of perchloric acid tetrabutylammonium or hexafluoro phosphoric acid tetrabutylammonium, by using anode formed with platina plate and cathode formed with platina wire.