It is stipulated in the Code of Design and Construction of Breakwaters（JTJ 298—1998）that the stable weight of armor blocks positioned 1 to 1.5 times of H13% under the LWL should be 1/5 of the calculating weight by Hudson formula.Model tests for a breakwater with water depth of 30 m show that there are some problems in applying the rule.To examine it,two model tests are conducted：one is armor blocks positioned 1 times H13% under the MWL,and the other is 1.5 times.Using the concept of “safety factor” to analyze the test data,we obtain the value of 2.43 and above 2.46 respectively,both of which are larger than 22.It manifests that the rule mentioned above is effective at H/d≥0.3 but failures at H/d=0.16.