Introduction; Cognitive impairment is among the most disabling features of Parkinson's disease (PD). Calcium has been postulated to play a role in PD. Aims; To evaluate the relation between serum calcium disturbances and deterioration of cognitive function in patients with PD. Methods; A case control study was conducted on 30 patients with clinically definite idiopathic PD diagnosed according to the Movement Disorder Society Clinical Diagnostic Criteria for Parkinson's disease and 30 healthy control subjects. The cognitive function was assessed in the participants using Mini-Mental State Examination (MMSE), Montreal Cognitive Assessment (MoCA) , Rey Auditory verbal learning test (RAVLT) and Visual reproduction subtest of the Wechsler Memory Scale Revised (VR WMS-R).Serum calcium level was measured for all the participants. Results; The PD patients were found to have a significantly lower level of serum total and ionized calcium than control subjects (P < 0.05). The PD patients with hypocalcaemia had a significant impairment of cognitive function in comparison with PD patients with normal calcium level regarding to the scores of both groups in MoCA, RAVLT and VR WMS-R scales ( P = 0.001 in all parameters) and MMSE (P = 0.002) There were significant positive correlations between serum calcium level and PD patients' scores of MoCA, MMSE, RAVLT and VR WMS-R (P = 0.001 in all parameters). Conclusion; The disturbances of serum calcium level especially low serum calcium level could deteriorate cognitive function in PD patients.