The purpose of this study is to investigate the impact behavior of composite pressure vessel (CPV) in-service case. Additionally, graphene nanoplatelets (GnPs) were introduced to epoxy resin. [±55°]4 basalt/epoxy specimens were manufactured using the filament-winding method, and the low-velocity impact (LVI) response was investigated. The LVI tests of CPVs were performed at energy levels of 2.5, 5, 7.5, 10, 15, 20, and 25 J under 50 bar internal pressure. The prestress value considering the internal pressure used under service conditions was taken as 1:5 of the burst pressure of the basalt/epoxy CPVs. After the LVI tests, the contact force–time and force–displacement curves were acquired. Also, absorbed energy values by CPVs were calculated over the obtained curves. The effects of GnPs on damage formations on basalt/epoxy CPVs under LVI loads were evaluated based on microscopic analysis. According to evaluations, damage formations such as matrix cracks on outer and inner surfaces of CPV, transverse cracks, and delamination were detected. Test liquid was detected on the impact surface at 25 J LVI test. The leakage observed due to the 25 J impact energy proves that damages can result in a leakage path. Consequently, the areas of damage formed in the cross sections perpendicular to the axis of basalt/epoxy CPVs decrease with the addition of GnPs, and the CPVs are more resistant to LVI effects.