A micro passage cold plate device for a liquid cooling radiator includes a upper cover and a lower plate. The upper cover has a working medium inlet at a side thereof and a working medium outlet at another side thereof. The inlet and outlet are trumpet-shaped such that the working medium expansively enters the cold plate gradually and leaves the cold plate with a reduced way gradually. Hence, the cold plate provides an even distribution of temperature, a lower thermal resistance and a better heat dissipation performance such that the stability of the two-state flow of the working medium is enhanced for heat dissipation device in the field of electronic field.