Objective: We present molecular cytogenetic characterization of mosaic small supernumerary marker chromosome (sSMC) derived from chromosome 17. Materials and Methods: A 43-year-old woman underwent amniocentesis at 17 weeks of gestation because of advanced maternal age. Amniocentesis revealed a karyotype of 47,XY,+mar[12]/46,XY[15]. Parental karyotypes were normal. Array comparative genomic hybridization (aCGH) and metaphase fluorescence in situ hybridization (FISH) were applied on cultured amniocytes. Quantitative fluorescent polymerase chain reaction (QF-PCR) was applied on the DNAs extracted from cultured amniocytes and parental bloods. The parents elected to continue the pregnancy. Conventional cytogenetic analysis on peripheral blood of the neonate was performed at age 2 months and 11 months. aCGH was performed on the peripheral blood at age 11 months. Results: aCGH on cultured amniocytes revealed a result of arr 17q11.1q11.2 (25,372,965–27,725,134)×3.2 (Log2 ratio = 0.73) compassing NOS2, POLDIP2, NEK8, and TRAF4. Metaphase FISH analysis revealed a result of +mar .ish der(17)(D17Z1+, wcp17+)[4/5]. QF-PCR assays excluded uniparental disomy 17. The marker chromosome was the sSMC(17) of der(17)(:p11.1→q11.2:). A 3004 g male baby was delivered at 38 weeks of gestation. Ventricular septal defect, neonatal developmental delay and speech delay with language problems were noted at neonatal follow-ups. The peripheral blood at age 2 months had a karyotype of 47,XY,+mar[11]/46,XY[29]. The peripheral blood analysis at age 11 months revealed a karyotype of 47,XY,+mar[27]/46,XY[13] and the aCGH result of arr 17q11.1q11.2 (25,616,440–27,822,571)×2.5 (Log2 ratio = 0.34). Conclusion: aCGH is useful in the precise measurement of the involved size of the euchromatic material and the associated genes in prenatally detected sSMC.