Multiwalled carbon nanotubes (MWCNT) have a fibrous structure similar to asbestos, raising concern that MWCNT exposure may lead to asbestos-like diseases. Previously we showed that MWCNT translocated from the lung alveoli into the pleural cavity and caused mesothelial proliferation and fibrosis in the visceral pleura. Multiwalled carbon nanotubes were not found in the parietal pleura, the initial site of development of asbestos-caused pleural diseases in humans, probably due to the short exposure period of the study. In the present study, we extended the exposure period to 24 weeks to determine whether the size and shape of MWCNT impact on deposition and lesion development in the pleura and lung. Two different MWCNTs were chosen for this study: a larger sized needle-like MWCNT (MWCNT-L; l = 8 μm, d = 150 nm), and a smaller sized MWCNT (MWCNT-S; l = 3 μm, d = 15 nm), which forms cotton candy-like aggregates. Both MWCNT-L and MWCNT-S suspensions were administered to the rat lung once every 2 weeks for 24 weeks by transtracheal intrapulmonary spraying. It was found that MWCNT-L, but not MWCNT-S, translocated into the pleural cavity, deposited in the parietal pleura, and induced fibrosis and patchy parietal mesothelial proliferation lesions. In addition, MWCNT-L induced stronger inflammatory reactions including increased inflammatory cell number and cytokine/chemokine levels in the pleural cavity lavage than MWCNT-S. In contrast, MWCNT-S induced stronger inflammation and higher 8-hydroxydeoxyguanosine level in the lung tissue than MWCNT-L. These results suggest that MWCNT-L has higher risk of causing asbestos-like pleural lesions relevant to mesothelioma development.