Thermoset matrix composites exhibit severe problems related to their environmentally unfriendly disposal and, therefore, there is a growing need to recycle the fibres and potentially reuse them in new products. The most common technique involves the removal of the polymer matrix at high temperature. The aim of the present work is to assess the evolution of mechanical and nanomechanical properties of single basalt fibres as a function of thermal exposure from the microscale down to the nanoscale, with a view to highlighting the mechanisms responsible for the observed decrease in tensile strength with increasing temperature. As-received and thermally treated basalt fibres have been characterized in terms of elastic modulus and tensile strength. Possible changes in the strength-flaw relationship during heat treatment have been investigated measuring the fracture toughness of the fibres by combining single edge notch tension and micro-pillar splitting methods along with high-speed statistical nanoindentation mapping over the fibre cross-sectional area. The fracture process of as-received and thermally treated basalt fibres appears to be controlled by surface flaws irrespective of heat treatment temperature, but the bulk properties (Elastic modulus and KIC) change during thermal recycling. Both Elastic modulus and KIC were found to increase after thermal exposure, with the occurrence of a significant microstructural anisotropy mainly localized in the outer layer of the fibers.