Silver nanoparticles were prepared by chemical reduction. Fusarium graminearum was used as the test strain. To study the inhibition of F. graminearum by silver nanoparticles, we studied the activities of protective enzymes superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), and the contents of osmotic adjustment substances soluble protein, soluble sugar and malonaldehyde (MDA) in F. graminearum. Silver nanoparticles inhibited F. graminearum and the inhibitory effect was increased with the concentration of silver nanoparticles. The inhibition rate of 10 μg/mL silver nanoparticles was more than 90% and EC50 was 0.59 μg/mL. When the treating time prolonged (2, 4, 6, 8 and 10 h), the activity of SOD, CAT and POD increased firstly and then declined. SOD, POD and CAT reached the maximum at 4 hours, and decreased to minimum at 10 hours. Silver nanoparticles also increased the MDA content and reduced the soluble sugar and protein contents in pathogens. These results indicated that cell integrity was destroyed in the presence of silver. This may be one of the inhibiting mechanisms of silver nanoparticles on the growth of F. graminearum.