DNA-functionalized gold nanoparticles (AuNPs) are one of the most commonly used reagents in nanobiotechnology. They are important not only for practical applications in analytical chemistry and drug delivery, but also for fundamental understanding of nanoscience. For biological samples such as blood serum or for intracellular applications, the effects: of crowded cellular proteins and nucleic acids need to be considered: The thermodynamic effect of crowding is to induce nanoparticle aggregation. But before such aggregation can take place,. there Might also be a depletion repulsive barrier. Polyethylene :glycol (PEG) is one of the most frequently used polymers to mimic the crowded cellular environment. We show herein that while DNA-functionalized AuNPs are very stable in buffer (e g, no PEG) and citrate capped AuNPs are very stable in PEG, DNA-functionalized AuNPs are unstable in PEG and are easily aggregated. Although such aggregation in PEG is mediated by DNA, no sharp melting transition typical for DNA-linked AuNPs is observed. We attribute this broad melting to depletion force instead of DNA base pairing. The effects of PEG molecular weight, concentration and temperature have been studied in detail and we also find an interesting PEG phase separation and AuNP partition into the water-rich phase at high temperature.