Background: In Radiation Therapy, the sparing of normal tissues can be performed using either multi-leaf collimators or Cerrobend blocks. The current work focuses on the PHYSICAL CHARACTERISTICs of Cerrobend blocks including attenuation coefficient, effective penumbra width and isodose curves undulation in penumbral regions. Materials and Methods: All measurements were performed using a dual energy linac and the Cerrobend blocks designed and fabricated using a commercial Cerrobend material. Data were collected using a calibrated ionization chamber as well as EDR2 films. Results: The results showed that the attenuation coefficient was found to be 0.4475 and 0.4276 cm-1 for photon beams 6MV and 15 MV, respectively, and a potential air bubble with a diameter greater than 3 mm affects beam attenuation significantly. The optimum Cerrobend block width was found to be around 16 mm. The isodose curves scalloping achieved for secondary collimator jaws, were also similar. Conclusion: If Cerrobend blocks are used as a basic method to protect normal tissues, its PHYSICAL CHARACTERISTICs will be recommended to be taken into account comprehensively.