Abstract: The present invention provides straight-chain crystalline polyuretdione which has a structure represented by formula (1): ##STR1## wherein Y represents ##STR2## X represents ##STR3## or --NCO; R.sub.1 is R.sub.2 or ##STR4## R.sub.2 is a residual group other than diol; R.sub.3 is a residual group obtained by removing an active hydrogen from a compound having one active hydrogen; R.sub.4 is a residual group obtained by removing two NCO groups from diisocyanate; and n is 1 or more,in formula (1), Y being replaced with a triazine ring represented by ##STR5## in the range of 0 to 15%, the polyuretdione having a latent NCO group content of 12 to 21% by weight, a weight-average molecular weight of 2000 to 20000 and a melting point of 60.degree. to 140.degree. C., and being not blocked at 40.degree. C. or less. The straight-chain crystalline polyuretdione of the present invention can be used as a hardener for powder coatings and does not cause a bad smell due to a blocking agent during baking of coating.