Psychological diseases are currently some of the most common diseases, as a result of many organic and social factors. Those diseases affect man and his mental and physical powers. This effect may extend to deficient or no perception and will that makes man unable to control his acts and, accordingly, can't be punished for the crimes that he commits while under the effect of those diseases, because they bar penal liability and reduce it, in accordance with the conditions of each of them. On this basis, Omani legislature regulated penal liability of this category in line with the modern regulation adopted by most world countries. The study elaborated liability of psychopath, and presented as preface the subject of penal liability in general by indicating its definition, basis, elements and conditions. Chapter one included the subject of psychological disease through its definition, symptoms, reasons and types. Afterwards, the researcher linked psychological disease with penal liability, and how psychological diseases affect the elements (perception and will) of penal liability, and how to prove psychological disease as a tool that helps judiciary to determine penal liability by identifying the type and severity of disease on the patient, and the patient's liability for the patient's acts. In the last chapter, the researcher defined the role of psychological disease in denial of penal liability which is denied in case the accused loses perception and will at the time when the incident is committed as a result of psychological disease. Conditions of this case and penal status of those to whom it applies were illustrated. The researcher addressed alleviated liability, which requires the accused's suffering of deficient or poor perception and will at the time of coming the crime. The researcher defined its conditions and penal status. Afterwards, the researcher handled the effect of psychological disease on the proceedings of the case, represented in suspension of procedures whenever the accused suffers a gross disease that makes him unable to control his acts at the time when the punishment is enforced until the accused comes back to his senses. At the end, the researcher addressed the impact of psychological disease on civil liability which is not affected by lack of perception and disorientation and continue to exist. The researcher presented some judgments that are related to the subject of the study to apply the theoretical part of the study to practical reality. The researcher mainly applied the inductive methodology of research to present the subject of the study. He also applied the comparative methodology in some parts to draw some conclusions that produced number of recommendations that we hope they can contribute to the legislative system of the Sultanate.