Abstract: With coefficient of inbreeding being 12.5-25%, the M and R line duck which lay blue shelled egg was applied to test-cross with egg shell N line.As a result, both blue egg shell homotype A and heterotype B have been obtained.While crossing A type with N line, all their full F1 duck laid blue egg.According to probability analysis and principle of qualitative character determination, it is of 95- 99% reliability for A type parents possessing homozygotle of dominant blue shell.Crossing B type with N line, full F1 duck laid both white and blue egg, so B type is a dominant heterozygote in terms of the mentioned principle above.Moreover, F1 from A cross N line was carried out back-cross with N line, the ratio of blue to white shell in F2 egg was 1:1, as self-crossing among F1 which become 3:1.And all full duck bred from either selfed A type or back cross between A selfed progeny and N line laid entirely blue egg.All the results mentioned above conform to the genetic law of qualitative character and elucidate that egg shell colour of Putian black duck is controlled by alleles with dominant blue and recessive white, therefore, blue shell dominanted over the white one.