[Objectives]  To investigate pre-copulatory mate choice in the invasive fall armyworm, Spodoptera frugiperda, and thereby both answer some fundamental biological questions and improve the effectiveness of releasing sterile insects to control this widely distributed, migratory, insect pest. [Methods]  We investigated whether several morphological traits (body weight, body length, forewing length and abdominal width) are involved in pre-copulatory mate choice under two operational sex ratios; 2♀1♂ and 1♀2♂. [Results]  The body lengths of females that were preferred by males were significantly shorter than those of less preferred females (t=3.20，P=0.006 9). However, none of the four measured morphological traits differed significantly between preferred and and unpreferred males (body weight: t=1.49，P=0.150 0; forewing length: t=0.59，P=0.557; body length: t=0.43，P=0.668; abdominal width: t=0.26，P=0.795). The primary morphological trait influencing mate choice was abdominal width (r=0.59, P=0.000 1), and the next most important was body length (r=0.36, P=0.027 1). Males, but not females, exhibit allometric growth, which significantly affects the relationship between body weight and the other three morphological traits (body length vs body weight: r = 0.07, P = 0.564; forewing length vs body weight: r = 0.14, P = 0.284; abdominal width vs body weight: r = 0.10, P = 0.437). [Conclusion]  Adult male S. frugiperda prefer to mate with females that have a relatively short body. The most important morphological trait influencing mate choice is abdominal width.