The discontinuous and continuous precipitation behavior, and the interaction between them during the aging process in a Cu-20Ni-20Mn alloy have been investigated in the temperature range 573 K-698 K by scanning electron microscope (SEM), transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). The results show that discontinuous precipitation is the predominant mechanism at 623 K and below, whereas both discontinuous and continuous precipitations occur when the temperature is higher than 673 K. The discontinuous precipitations with a lamellar mixed structure, consisting of alternate lamellae of matrix and NiMn phase, nucleate and grow preferentially on grain boundaries during the aging process. Continuous precipitations can either partially or completely inhibit the growth of discontinuous precipitation colonies. Obvious inhibition action on the migration of reaction front of discontinuous precipitation colonies are found by experimental results, owing to the continuous precipitates when their diameters reach up to about 5 nm or larger. The grain boundary chemical diffusion triple product sδDb is calculated according to an Aaronson-Liu model. In addition, the activation energy for discontinuous precipitation is determined to be in range between 106.7 to 113.5 kJ/mol based on Arrhenius equation, well below the activation energy for volume-bulk diffusion of Ni and Mn in Cu-rich alloy, indicating that discontinuous precipitation is easy to occur in grain boundaries.