Fusion - fission hybrid reactor consists of two main parts. The first part is the Dense Plasma Focus (DPF) where the fusion process is performed and the second part is the blankets which surround the DPF and the fission process is done in the blankets. The most of the fusion neutrons, near the fusion source, are directed upward, so the blanket over the DPF is designed in forms cylindrical, spherical and frustum cone of volumes ۱۸۳,۳۴۷ cm۳, ۱۳۶,۳۷۸ cm۳ and ۵۸,۲۲۳.۲ cm۳ respectively at the same effective multiplication factor. Then the thermal neutron diffusion equation, buckling and neutron parameters are studied. After selecting the first blanket, the second blanket is designed in forms cylindrical, spherical and frustum cone of volumes ۴.۵۰۰ × ۱۰۶ cm۳, ۳.۶۸۹ × ۱۰۶ cm۳ and ۶.۴۴۱ × ۱۰۵ cm۳ respectively. So the frustum cone is chosen for the first and second blanket. The neutron return probability in the frustum cone from the second blanket to the first blanket is low, so the first blanket&#۳۹;s effective multiplication factor does not change and the reactor&#۳۹;s total effective multiplication factor is stable. Also, MCNPX code has been used to checkout and select neutron multiplier and reflector cells for the most neutron yield