Purpose To examine the validity of the Gross Motor Function Measure (GMFM) in assessing gross motor function (GMF) of extremely preterm infants relative to the Alberta Infant Motor Scale (AIMS). Methods The retrospective study included 60 extremely preterm infants born at <28 weeks gestational who were assessed for gross motor function using GMFM88 and AIMS at 6 and 12 months of corrected age (CA). The correlations between the AIMS raw scores and the total scores of both GMFM versions at 6 and 12 months CA were assessed. Results GMFM66 and GMFM88 total scores showed moderate to strong correlation with AIMS (p <0.001). GMFM66 and GMFM88 scores were significantly different between infants who received AIMS scores above and below the 10th percentile (p < 0.05). Conclusions The GMFM had good validity in assessing GMF in extremely preterm infants at 6 and 12 months CA.