Objective: To identify the presence of cancer stem-like cells (side population,SP) in thyroid cancer and compare the growth pattern,clonogenity and invasive potential between cancer SP and non-SP cells. Methods: Three different human anaplastic thyroid cancer cell lines(including HTh74,SW1736,and C643) were stained with Hoechst 33342 and sorted for a small fraction of SP by fluorescence-activated cell sorting(FACS). The gene expression of stem cell marker-Oct4(octamer-binding transcription factor 4) and genes related to cancer resistance and relapse-ABCG2 (ATP-binding cassette superfamily G member 2) and MDR1 (multidrug resistance 1) in SP and non-SP cells was compared by performing RT-PCR and immunofluorescent staining. Colony formation assay was performed to evaluate the self-renewal potential of SP cells,and cell invasive potential was measured by using the Matrigel invasion assay. Results: Cancer SP cells possessed some intrinsic stem cell properties as they preferentially expressed stem cell marker Oct4 and ABC transporter genes-ABCG2 and MDR1,and can generate both SP and non-SP cells in vitro. Colony formation analysis and Matrigel invasion assay revealed that these cancer SP cells were more clonogenic and displayed higher invasive potential than non-SP cells. Conclusion: These findings suggest that the SP in thyroid cancer cell lines is enriched with stem-like cancer cells that may be the source of tumor drug resistance and recurrence.