The present invention relates to an evaporator, and more particularly, to an evaporator which can restrict a surface area of a communication hole with respect to a cross sectional area of a compartment and a surface area of a tube with respect to a surface area of a fin, thereby providing a dimensional extent for maximizing the heat exchange efficiency. Therefore, by optimizing a relation between the surface area of the communication portion and the surface area of the compartment of the first header tank and dimensions for each surface area and the heights of the tube and fin, the present invention provides a dimensional extent for maximizing the heat radiation amount, reducing the maximum temperature deviation of the core portion and allowing the refrigerant and air to be smoothly flowed, thereby maximizing the heat exchange efficiency.