Nanoscale understanding of high-temperature crystal growth dynamics in solution has been a challenge to be tackled by many researchers engaged in investigating solution processes for bulk single crystal growth. Here we propose a new approach to in situ observation at a buried solid/liquid interface in high-temperature solution using a conventional confocal laser scanning microscope. In the solution growth of 4H-SiC with Si-Ni based alloy flux as a model system, we show the ability to quantitatively analyze step motions at the growing SiC crystal on the nanoscale at high temperatures up to 1700 °C in a vacuum. The temperature-dependent step-advance rates for various steps with different step heights demonstrated the advantageous effect of adding Al to the flux on the step-flow growth of SiC: addition of just 4 at% Al effectively suppressed step-bunching. These experiments point to the importance of in situ nanoscale observation in understanding solution growth mechanisms, and hence the potential to accelerate the development of solution growth processes for high-quality bulk single crystals.