Cancer is one of the most killing diseases in this century. The available cancer treatment now is by radio-therapy and chemo-therapy. Both of them cause very harmful side effects on healthy tissues. These side effects lead to decreasing the drug dose and delaying the therapy. Nano-robots can be used to deliver the drug to only tumor cells without harming the healthy tissues. In this paper, a new modification in PSO algorithm is proposed to facilitate Nano-robots delivery to the cancer area. The modified algorithm is Directed PSO (DPSO). It can efficiently deliver all Nano-robots to the target area in a very short time.