Asteraceae family plants are receiving great attention because they have potential anticancer activity. Therefore, in this study, the cytotoxic effect of Conyza canadensis (L.) Cronquist (Asteraceae) were tested against human lung adenocarcinoma cell lines (A549 and H1299) for the first time. Cytotoxic effect of the n-hexane, chloroform, n-butanol and remaining water (R-H2O) extracts fractioned from the methanol extracts of the aerial parts and roots of C. canadensis was investigated using Sulforhodamine B (SRB) assay and percent (%) viability was measured. The results indicate that the extracts of C. canadensis have cytotoxic activities on these cells in a dose-dependent manner. The root extracts exhibited relatively higher cytotoxic effects than the aerial parts of the plant. The most active extract was found to be n-hexane extract of the roots with IC50 values 94.73 and 84.85 µg/mL on A549 and H1299 cell lines, respectively. These results suggest that C. canadensis exhibits moderate cytotoxic effect in lung cancer cells. This might be taken into account in its use for therapeutic purposes.