Introduction: Multileaf collimator (MLC) is state of the art among the radiation field shaping systems for conformal radiotherapy techniques. Some advantages of this system are as follow: reduction of treatment time, higher precision in conforming the radiation field to the projection of the target volume and lowering the chance of errors made by technologists. The purpose of this research was to design and construct the mechanical section of a prototype MLC. 
Material and Methods: The MLC consists of a base plate, 52 leaves, 2 leaf carriages, 2 nuts and 52 triangular thread screws. The leaves are arranged in two opposing banks each containing 26 leaves. The carriages are used to extend the movement of the leaves. The components of the MLC are fabricated by machining processes. The MLC is designed to be mounted on the conventional linear accelerators as an add-on accessory. The leaves were made of aluminum. The dimensions of the leaves are 5 x 200 x 50 mm. 
Result: The collimator system weighs approximately 37kg excluding the leaves. The projected width of each leaf on the isocenter of a Varian linear accelerator is 10 mm. The movement of the leaves is accomplished by using the triangular thread screws. The screws required to have a low axial backlash which in turn results in lower precision positioning of the leaves. The positioning accuracy of each leaf is approximately 1.4 mm.
Discussion and Conclusion: Mashhad MLC system, in it’s present form and design weighs 37 kg. but if Aluminum blades are replaced by tungsten ones then the over all weights of MLC may exceed the weight limit a linac may hold. Therefore futher modifications are required to reduce the MLC total weight. It is desirable to use ball screws and roller cages instead of triangular thread screws to achive a higher positioning pereision of the blades