Cyanobacteria Hapalosiphon species are able to synthesize secondary metabolites which could inhibit seed germination and root growth of some plants. It has been reported that the Hapalosiphon extract can reduce seed germination, shoot and root growth, root cell division and cell viability in some plants. This study aimed to investigate seed germination, shoot length, root length and cell viability of plant roots (Chinese cabbage, rice, giant mimosa and toi ting) that were exposed to crude methanolic extract of the Hapalosiphon at 0, 5, 10, 15 and 30 g DW L-1. The results showed that the extract had no effect on seed germination of all the plants. The shoot length of Chinese cabbage and giant mimosa was not inhibited whereas that of rice and toi ting showed signifi cantly decreased at the highest concentration when compared with the control. The extract inhibited root length, causing it to be less than 50% of the control in all tested the plants when exposed to the highest concentration. Cell viability of plant root tips were studied by Evans blue staining method. Giant mimosa and toi ting root tips signifi cantly lost their cell viability when treated with the extract. These results indicated that although the Hapalosiphon extract inhibited root length of all the tested plants, it caused cell viability loss in root tips only in giant mimosa and toi ting.