Ulva linza is one of the species that causes green tides in the Yellow Sea,China.Due to the difficulties in chromosomal preparation,the large numbers of chromosomes,and their relatively small sizes,there have been no reported studies on Ulva macroalgae chromosomes.The karyotypes and chromosomes in U.linza were observed after a series of treatments.The chromosomes were pretreated with 0.1％colchicine for 12 h and then mixed with enzymes.The samples were dropped from 30 cm height onto glass slides,which spread out the surface coat.These pretreatments were the optimal chromosomes preparation treatments.The prepared chromosomes were stained with 4',6-diamidino-2-phenylindole(DAPI),which is a fluorescent probe sensitive and specific to DNA.The chromosome number in the haploid male and female gametophytes was n=9,and was 2n=18 in the diploid sporophytes.The female gametophyte chromosomes were between 0.804 and 2.292 μm in size,the male gametophyte chromosomes were between 0.917 and 2.916 μm,and the sporophyte chromosomes were between 0.912 and 2.167 μm.The relative sizes of the chromosomes were used to analyze the karyotypes of the female and male gametophyte chromosomes.The results provide a solid foundation for the basic technique that can be used to localize molecular markers of Ulva chromosomes.