Novel highly pure and stable crystals of .beta.-nicotinamide-adenine-dinucleotide tetrahydrate (NAD) which is triclinic system and has a space group of P1 or P1 and lattice constants: a=8.861 .ANG., b=11.181 .ANG., c=8.630 .ANG., .alpha.=90.82.degree., .beta.=103.40.degree. and .gamma.=109.71.degree.. The crystalline NAD is prepared by cooling a 20 to 60 w/v % aqueous solution of amorphous NAD, which has preferably been treated with a porous weakly basic anion exchange resin to remove impurities, at a temperature of 0.degree. to 20.degree. C. When the crystalline NAD is added to the aqueous solution as seeds, the desired high pure crystalline NAD is prepared without conducting the treatment of amorphous NAD with the porous weakly basic anion exchange resin. A high pure amorphous NAD is obtained from the crystalline NAD by dissolving the crystalline NAD in water and subjecting the aqueous solution to freeze drying or precipitation with a solvent.