Nitric oxide (NO) is free radical gas with biological activity in signal transmission. NO mediated various responses to biotic or abiotic stress and played an important role in phytoplankton growth and stress-resistance, which are similar to higher animals and plants. Additionally, NO and reactive oxygen species (ROS) function as a signal to initiate programmed cell death (PCD) in phytoplankton. The review summarized the NO pathway, the methods measuring NO and its application, the advantages and disadvantages of different measurements, the physiological function of NO, the relationship between NO and PCD, and the function of NO as a signal molecule in phytoplankton. Finally, some ideas concerning nitric oxide research in organisms were put forward.