Currently used humidity sensors have a large size which seriously hinders their applications in many special fields. Therefore, a facile way for fabricating miniaturized humidity sensors is desired. In this work a high-performance, nanofiber bundle sensor (NfBS) was fabricated through immersing nylon66 (PA66)/multiwall carbon nanotubes (MWCNTs) nanofiber bundles into a polyvinyl alcohol solution. In the PA66/MWCNTs nanofiber bundles the MWCNTs were absorbed on the surfaces of the PA66 nanofibers. The NfBS exhibited a wide range of 11-85% relative humidity (RH) sensitivity with a fast response-recovery time as well as small hysteresis and good repeatability, but had an extremely small size (about 0.1 mm in diameter and 2 mm in length) due to the unique advantages of nanofiber bundles. In addition, the NfBS showed good performance in monitoring human breath. The results in this study provide a promising opportunity and a good method for manufacturing mini-humidity sensors with smaller sizes for electronic products.