The current design standard for port structures specified a criterion for assessing liquefaction potential using the grain size accumulation curve and the N-value of the Standard Penetration Test. This criterion has been harmonious, in predicting liquefaction as a whole, to the liquefaction case histories of the large earthquakes after the 1968 Tokachi-oki Earthquake. However, this criterion has a following difficulty in practice; this criterion could not predict vertical range of soil layers which likely liquefy under strong ground motions. Therefore, taking the newly obtained case histories and the recent progress in this field into consideration, an extention of this criterion was studies so as to be able to predict vertical range of the liquefiable layeres and a new criterion was proposed.
　Several researchers have proposed their criteria for assessing liquefaction potential. Comparisons of the new criterion with other criteria were also made.