This paper reports the synthesis of metal-organic framework (MOF) composites based on MOF HKUST1 and Al2O3 nanowhiskers (NWs), functionalized with different ratios of (3-aminopropyl)triethoxysilane (APTES). The effect of the Al2O3 NW content on the properties of the MOF HKUST1 composites was also reported. The synthesized materials were shown to be real composites and not a physical mixture of the constituents. In addition, the high Al2O3 content in the MOF HKUST1 matrix hampered the thermal and textural properties of the composite. This behavior may be related to the effects of APTES NWs on composite synthesis. The oxides affected the crystal growth of MOF HKUST1, changing the crystallinity of the composites. However, when the optimal amount of APTES NWs was used for the composite synthesis, i.e., 10% w/w (copper and APTES NWs), the main characteristic features of the MOF were retained by the composites, as demonstrated by X-ray diffraction, thermogravimetric analysis, and CO2 uptake at 298 K.