Gold nanorods (AuNRs) are promising nanoplatforms for cancer photothermal therapy due to their capability to absorb light in the near-infrared (NIR) window. This work was conducted to evaluate the DNA damage in Ehrlich tumor tissues associated with photothermal therapy mediated with very small sized pegylated gold nanorods (PEG-AuNRs). The accumulated gold nanorods inside tumor tissues were photo-excited with 300 mW NIR laser for 50 min. The physical properties of PEG-AuNRs were investigated using transmission electron microscopy, size distribution measurement, and UV-vis absorption spectroscopy.  The efficacy of the in-vivo photothermal therapy of intravenously injected PEG-AuNRs was assessed using tumor size measurements. Also, oxidative stress and comet assay were performed to measure the percentage of DNA damage. The results showed that PEG-AuNRs were successfully prepared with a length of 8 nm and a longitudinal absorption peak at 800 nm. FTIR study confirmed the successful coating of polyethylene glycol on the surface of the gold nanorods. The in vivo experiment showed that PEG-AuNRs elicited tumoral growth arrest by producing free radicals in addition to its photo heat conversion which was sufficient to induce cellular damage by causing DNA damage