This study examined the influence of internal capabilities and external networks on firm performance by using data from 137 Korean technological start-up companies. Internal capabilities were operationalized by entrepreneurial orientation, technological capabilities, and financial resources invested during the development period. External networks were captured by partnership- and sponsorship-based linkages. Partnership-based linkages were measured by strategic alliances with other enterprises and venture capitalists, collaboration with universities or research institutes, and participation in venture associations. Sponsorship-based linkages consisted of financial and nonfinancial support from commercial banks and the Korean government. Sales growth indicated the start-up's performance. Regression results showed that the three indicators of internal capabilities are important predictors of a start-up's performance. Among external networks, only the linkages to venture capital companies predicted the start-up's performance. Several interaction terms between internal capabilities and partnership-based linkages have a statistically significant influence on performance. Sponsorship-based linkages do not have individual effects on performance but linkage with financial institutions has a multiplicative effect with technological capabilities and financial resources invested on a start-up's performance. Implications and directions for future research were discussed.