PURPOSE: To evaluate the dose under lung block as a function of depth and the effectiveness of a block as a function of block width. MATERIALS AND METHODS: Field size of mantle field was 22.8 x 32.4 cm2 . Dose distribution of the mantle field was measured with two dimensional water phantom system. To analyze the effectiveness of the lung block, central axis plane, 5cm off-axis plane, and 10cm off-axis plane were studied. RESULTS: The dose under the lung block was recorded with maximum at the depth between , 5cm and 10cm . In the central axis plane, dosimetric block width was 10-15% les than physical block width. In the 5cm off-axis plane. Dosimetric block width was 4-9% less than physical block width. In the 10cm off-axis plane, dosimetric block width was 2% less than physical block width. CONCLUSION: Depth dependence of the dose under the lung block was founded. Also, block width dependence of the lung block was founded. To induce the accurate relation between the physical block width and the "effective" block width, it needs more detailed understanding of the variables involved.