Underweight is a nutritional status of toddlers characterized by a lack of weight based on anthropometric index weight-for-age. In Indonesia, the anthropometric index is recorded on a KMS (Card Towards Health) which refers to WHO-2005 Standard Growth Chart (WHO-2005 SGC). The toddler samples used by WHO-2005 SGC, i.e., from Ghana, Oman, Norway, the USA, Brazil, and India, are physically different from the Indonesian toddlers. This research aims to design Z-Score Standard Growth Chart (Z-Score SGC) of weight-for-age based on sample toddlers from East Java by using least square spline estimator in semiparametric regression model. The obtained Z-Score SGC would be used to assess the nutritional status of toddlers in East Java. Results showed that the highest average weight gain was achieved at interval of age 0 ≤ t < 6 for about 0.7045 kg, and growth of toddler boys was higher than girls with a difference of 0.4 kg. Also, Z-Score SGC gave a lower reference than WHO-2005 SGC such that percentage of malnutrition and less-nutrition status categories provided by Z-Score SGC was also lower than those by WHO-2005 SGC.