Current CAR therapies relying on viral vectors face challenges such as safety concerns, high costs, and long preparation times. mRNA-encoded CARs in NK cells offer a promising alternative, but the instability of linear mRNA limits its effectiveness. This study explored self-amplifying RNA (saRNA), and circular RNA (circRNA) to achieve sustained CAR expression and improved outcomes. Anti-CD19 CAR RNA vectors were electroporated into NK-92 cells, and CAR expression, viability, and cytotoxicity were assessed via flow cytometry. Cytotoxicity was evaluated in a 5-hour coculture assay with CD19-positive Nalm-6 and CD19-negative K562 cells at a 1:1 effector-to-target ratio. At day 1 post-electroporation, circRNA, and saRNA achieved comparable CAR expression (80.3% and 85%, respectively). However, circRNA expression declined sharply to 3.1% by day 4. In contrast, saRNA maintained CAR expression above 85% from day 1 to day 3 (86.6%, 87.6%, and 91%, respectively) and remained detectable until day 7 (72.2% on day 4, declining to 20.4% by day 7). In cytotoxicity assays, CAR-NK-saRNA showed 41.2% cytotoxicity against Nalm-6 cells, outperforming CAR-NK-circRNA (39.6%) and unmodified NK-92 cells (11%). These results demonstrate that saRNA-based CAR-NK cells achieve sustained CAR expression and superior cytotoxicity compared to circRNA and unmodified NK-92 cells. While circRNA exhibited rapid decline, saRNA maintained detectable CAR levels for up to 7 days. Further studies are needed to optimize expression levels, evaluate cytotoxicity over extended periods, and test saRNA-based CAR-NK cells in vivo. Nevertheless, saRNA represents a promising non-viral alternative for CAR therapies, addressing key limitations of viral vectors. Financial support: FAPESP (2022/16140-5, 2013/08135-2, NPOP-Nutec 2020/07055-9), CNPq (442484/2020-8), and FINEP.