The growing usage of carbon nanotubes (CNTs) in different applications increases concern of negative influence of CNTs on human health. Therefore the control of CNTs is significant and filtration is an effective method to control airborne CNTs. We investigated penetration of airborne CNTs through nanofiber filters composed of micro and nanometer fibers. The theoretical model for nanofiber filter media was also investigated and the experimental results agreed with the model. The penetrations of CNTs through the nanofiber filters were compared with the penetration through a screen type filter made of stainless steel. Moreover, the geometrical lengths of CNTs were calculated by the filtration method using the nanofiber filter. The calculated lengths showed a good agreement with the measured lengths by scanning electron microscopy analysis. The developed method for the length measurement of CNTs provides the possibility of online measurement for elongated particles such as CNTs.