Ricin toxin is a disulfide-linked heterodimeric glycoprotein consisting of the toxic A chain and the cell-binding B chain that acts through the irreversible inactivation of eukaryotic ribosomes and inhibition of the polypeptide chain elongation, which finally leads to cell death. The toxin is extracted from castor plant beans (Ricinus communis) and it consists of 1–5% of the beans’ dry weight. Ricin is of concern in the area of biodefense because it is readily available to individuals or groups with little technical expertise or funding and its source is ubiquitous in the weed and in the cultivated crop in many countries. Since ricin can easily be distribute in the body, it has been classified by the CDC (US center for Disease Control and Prevention) as a class B biothreat. There has been much investigation on the diagnosis, treatment and production of a potent vaccine against this agent. For diagnosis of ricin poisoning in clinical cases, immunological techniques have been used for screening of samples and combination of high tech chromatographic and mass spectrometric methods could be used for poisoning confirmative analysis. There is currently no specific treatment available for poisoning with ricin but two closely related RTA-based subunit vaccines, contain RiVax and RVEc, are now under development. The aim of this study is presentation of ricin toxin, diagnosis methods, treatment and new vaccine candidates against it. Sadeghi D, Ebrahimi F, Zeinoddini M, Hosseini SA, Hedayat Manesh Z.Ricin Toxin; Mechanisms of Toxicity, Diagnosis Methods, Treatment and Immunization Against it.Jundishapur Sci Med J 2017;16(1):103-124.