This study is a single-center, parallel and double-blind study in Movement Disorders Clinic of Shohadaye Tajrish Hospital. Patients, researchers (physicians, outcome assessors) and data analysts are blinded. After assessing the inclusion and exclusion criteria's, patients who assigned the informed consent form, are randomly divided into control and treatment groups.Patients in treatment or placebo groups respectively receive mirtazapine (15 mg) or placebo, once a day for 12 weeks. Primary (anxiety) and secondary (depression, fatigue, sleep disorders, and quality of life) outcomes are evaluated at baseline, and after 4 and 12 weeks of treatments.

Inclusion Criteria:

• Men and women over 17 years old
• Patient with Parkinson's disease according to UKPDSBB criteria
• Patient with mild/moderate Parkinson's disease according to Hoehn and Yahr Scale (HY score = 1-3)
• Self-report or clinical diagnosis of anxiety
• Patients who have signed informed consent to participate in the study.

Exclusion Criteria:

• Pregnant and lactating women
• Parkinson's patients with onset of disease in less than 1 year
• Unstable medication for Parkinson's disease during the last two weeks
• Parkinson's patients with DBS
• Patients with other neurodegenerative diseases like multiple system atrophy (MSA), Huntington's and etc.
• Patients with major depressive disorder
• A history of using SSRIs, SNRIs, benzodiazepines and β-blockers during the last 4 weeks
• A history of using MAO inhibitors
• A history of alcohol and substance abuse
• A history of acute stress during the last 3 months
• A history of suicide
• A history of cardiovascular diseases
• A history of liver and or kidney disorders