Group A human rotaviruses (RV) are a leading cause of severe dehydration and gastroenteritis in infants and young children. A large body of evidence suggests that Lactobacillus rhamnosus GG (LGG) has an effect on the incidence and severity of acute RV-induced diarrhoea; however, the timing and dosage of LGG treatment remains controversial. In the present study, a neonatal mouse model with human RV-induced diarrhoea was set up and the pathophysiological characteristics of the animals were examined. Our results indicated that RV-infected mice developed diarrhoea, accompanied by increased secretion of intestinal mucosa sIgA and serum interferon (IFN)-γ, tumour necrosis factor (TNF)-α, as well as decreased serum IgA. In addition, epithelium vacuolation was noticed in the jejunum microvillus of RV-infected mice. After intragastric administration of low (2 × 105 CFU), middle (2 × 107 CFU) or high (2 × 109 CFU) levels of LGG for four consecutive days before or after RV infection respectively, the RV-infected mice showed a shortened duration of diarrhoea and decreased epithelium vacuolation in the jejunum. Administration of a high dose of LGG before the RV infection was found to have better protective effects against RV infection than other regimens. This study demonstrates that the protective effects of LGG against RV-induced diarrhoea are highly correlated with the timing and dosage of LGG administration in neonatal mice.