Biogas production is one of the key technologies by which different types of substrates can be transform into renewable energy products. Agricultural crops are often used in biogas production, and the most common is corn silage. Although its usage is the most efficient and technologically effective process, corn shows several negative aspects such as high environmental impact, competition with food industry and high initial investments. Arundo donax L. is an invasive perennial plant in Croatia and all over the world, and in recent years its potential for biogas production has been considered. High biomass production per ha, low investments and environmental safety as it does not require additional fertilization and tillage are the main advantages of this crop. The primary goal of this research is to determine the biomass characteristics of Arundo donax L. and the effect of harvest time on biomethane and biogas production. Samples were collected in November 2020. and March 2021. and tested in lab reactor for anaerobic digestion. It was found that harvest time affected the composition of biomass and ultimate biogas production. A. donax L. showed good performances, although more research and improvements in pretreatment technologies are needed. A. donax L. is a suitable crop for biogas production, especially because of all the advantages in comparison with corn silage.