The inoculum type and infection method are effecting of percentage and severity of common smut disease on corn .Result of Field study indicated that the type inoculum mixture (Teliospores and Basidiospores) for Ustilago maydis was the best inoculum to achieve highest infection for two periodes after 10 and 20 days of inoculation. Percentage of disease infection was 40% for both periodes and disease severity was 5.17 for both periodes too, compared to control (no treatment) was 6.7% and disease severity was 1. Result of method of inoculation indicated that injection of silk channel with suspension of (teliospores and Basidiospores) gave highest percentage of infection 40% after 10 and 20 days with disease severity 4.67. Regarding the study of sensitivity 10 foreign hybrids (wich cultured in wide area in Iraq). Result demonstrated that Abgaro and Dijla hybrids were more resistance to the disease. Percentage of disease sinfection were 16.7 and 16.7% on foliage respectively the percentage .However, percentage infection of ears were 23.3 and 26.7% and disease severity was 2.5 and 2.5 respectively. Also, result indicated that local corn variety was most sensitive among corn hybrid, percentage of infection was 33.3% and 40% on foliage and ears respectively, disease severity of ears was 5.33.