Hydrodynamic escape of atmosphere is crucial to understand the early evolution of rocky planets in our solar-system and beyond. The escape is generally thought to be driven by the net UV heating energy which is remaining energy among the heating by irradiated stellar UV photons, the heating of chemical reactions, and the radiative cooling. While hydrodynamic escape simulations have been developed for incorporating all these processes, the radiative cooling by molecules has been treated as emission from just selected lines or rotational/vibrational bands. This is because it is numerically difficult to consider all radiative lines of molecules due to the large number of lines (e.g., the number of H2O lines is about half a million in HITRAN database Rothman et al., 2013). Here, we show how efficiently correlated-k method, which is an efficient way designed to group molecules' lines for maintaing accuracy in a radiative transfer calculation (e.g., Liou1980) works with different wavelength resolutions (R). For instance, the method is widely used for lower atmospheres in hydrostatic condition (e.g., Showman et al. 2008). In our assessment, we consider Mars-size planets with H2-rich transonic atmospheres containing H2O or CO as radiative species and having different thermal structures and different fractions of the radiative species. We found that the error of the radiative cooling rate calculation with correlated-k method comparing to line-by-line method is within the order of ten percent for R>3000. Also, we found that correlated-k method with R=1000 is accurate enough to calculate the escape rate and temperature profiles because the differences of them between R=1000 and R=3000 are below 10 percent and 1 percent, respectively. Also, we discuss in what condition of planetary atmosphere correlated-k method efficiently works in hydrodynamic simulations.