The aim of this work was to investigate the effects of moderate vacuum conditions (10 Pa, 1 × 103 Pa and 2 × 104 Pa) on glass forming ability, thermal stability and aging behavior of Co42Cu1Fe20Ta5:5B26:5Si5 amorphous samples in terms of size and distribution of crystalline precipitates. The thinnest parts (300 μm) of the wedge-shaped samples were amorphous for all vacuum conditions, and they had similar compositional, structural and thermal characteristics. However, they represented different microstructural features after heat treatments at 950 K and 1100 K. The same phases precipitated in all samples but the amount and the size of the precipitates increased as the pressure was raised to the normal atmospheric value. The differences in the glass forming ability and the microstructure due to aging are considered to originate from lower thermal characteristics, such as relaxation and reduced glass temperature as well as γ parameter, resulting from the poor vacuum pressure used in the production process. The differences in the microstructures and hardness values caused by heat treatments have been observed.