In present study during 2016 and 2017, from 106 Fusarium graminearum isolates collected from three provinces including Mazandaran, Golestan and Ardabil, 15 isolates were isolated from infected corn and others from infected wheat heads. Fungal isolation was performed according to Nelson et al. (1983) method in Nash-Snyder, PDA and CLA culture media. MBM (Mung bean medium) and a medium containing wheat straw were used fungal sporulation. Identification of the isolates was carried out according to Leslie and Summerell (2006). The pathogenicity test of isolates on corn was carried out using Danielsen et al. (1998) method. For this purpose, corn seeds of B73 was used. Pathogenicity test of isolates in wheat was done according to the method of Mesterhazy (1978). Tajan cultivar, which is recognized as susceptible to the disease, was used for cultivation. The pathogenicity tests were performed based on completely randomized design with 3 replications on all isolates of fungi plus control in both plants including corn and wheat. The results of analysis of variance (ANOVA) showed significant differences in aggressiveness between the F. graminearum isolates at the level of 5% in both hosts. The pathogenicity variability of F. graminearum isolates from three provinces of Golestan, Mazandaran and Ardebil were high on corn and wheat. Although most of the isolates of F. graminearum were pathogenic to both hosts, wheat and corn, but the aggressiveness of the isolates was not the same for wheat and corn. In the comparison of mean values based on Duncan’s multiple range test at level of 5%, isolates were divided into 18 groups in terms of pathogenicity severity on corn, which consisted of 36, 41, 54, 57, 62, 64, 64, 60, 62, 56, 60, 52, 52, 53, 49, 46, 38 and 11 isolates. The isolates were classified into 33 groups of pathogenicity in wheat, which consisted of 33, 37, 39, 42, 43, 44, 45, 46, 45, 47, 47, 48, 48, 50, 50, 50, 50, 48, 48, 47, 41, 39, 35, 33, 33, 30, 1, 1, 2, 2, 3 and 5 isolates. Majority of the pathogenic isolates on wheat were also able to cause infection on corn, indicating that this cropping system not only is problematic for wheat, results in disease epidemic, but also it can threaten corn cultivation in infected areas. The findings of this study are important for adopting conservation agriculture scenarios in infected areas with respect to the management of plant residues.