Objective To discuss the changes of serum levels of Klotho and fibroblast growth factor 23 （FGF23） in the small for gestational age （SGA） infants after birth， and to clarify their relationships with growth and development. Methods A total of 35 SGA and 53 appropriate for the gestational age（AGA） infants were selected and divided into SGA group（n=35） and AGA group（n=53）， including 51 infants in premature group， among them 20 infants in preterm SGA group and 31 infants in preterm AGA group； among them 37 infants in full-term group， 15 infants in full-term SGA group and 22 infants in full-term AGA group. The clinical materials of the infants in various groups were collected. The levels of Klotho and FGF23 in serum and clinical biochemical markers of the infants on the 7th and 14th days after birth were detected. The relationships between the levels of Klotho and FGF23 in serum on the 7th and 14th days postnatally and newborn growth indicators such as body weight， body length， head circumference， chest circumference， and Kopu index， as well as their correlations with calcium and phosphorus metabolism were analyzed. Results Compared with AGA group， the body weight， body length， head circumference， chest circumference， and Kopu index of the infants in SGA group were decreased （P<0.05）. On the 7th and 14th days after birth， compared with preterm group， the serum levels of Klotho and FGF23 of the infants in full-term group were significantly increased （P<0.01）. Compared with the 7th day after birth，the levels of serum Klotho of the infants in preterm and full-term groups on the 14th day were significantly increased （P<0.01）， and the levels of FGF23 in serum were decreased （P<0.01）. Compared with AGA group， the levels of serum Klotho and FGF23 of the infants in SGA group on the 7th and 14th days after birth were significantly decreased （P<0.05 or P<0.01）. Compared with the 7th day after birth， the levels of serum Klotho of the infants in both AGA and SGA groups on the 14th days after birth were significantly increased （P<0.01）， and the FGF23 levels were decreased （P<0.05 or P<0.01）. Compared with preterm AGA group， the levels of Klotho and FGF23 in serum of the infants in preterm SGA group on the 7th and 14th days after birth were significantly decreased （P<0.05 or P<0.01）. Compared with full-term AGA group， the levels of Klotho and FGF23 in serum of the infants in full-term SGA group on the 7th and 14th days after birth were significantly decreased （P<0.05 or P<0.01）. In SGA group，the serum levels of Klotho and FGF23 on the 7th day after birth were positively correlated with the gestational age， body weight， body length， head circumference， chest circumference， and Kopu index （P<0.05 or P<0.01）； there was a positive correlation between the serum level of Klotho and the serum level of FGF23 （P<0.05）. In terms of calcium-phosphorus metabolism， in SGA group，the serum level of Klotho on the 7th day after birth was positively correlated with serum phosphorus level （P<0.01）， and the level of serum FGF23 on the 7th day after birth was positively correlated with serum calcium and phosphorus levels （P<0.05 or P<0.01）. Conclusion Klotho and FGF23 proteins are closely associated with growth and development and phosphate metabolism of the infants. The expression levels of Klotho and FGF23 in serum of the SGA infants postnatally are lower， but the secretion of Klotho is increased with the gradul improvement of each organ， and the decrease of FGF23 may be the adaptive response.