Effects of balloon drift information (i.e., position and elapsed ascent time of the balloon) on the assimilation of radiosonde observations are investigated by using the Weather Research and Forecasting (WRF) Model and its data assimilation (WRFDA) system. Special radiosonde observations over the Korean Peninsula, which include the exact position and elapsed time of the balloon, are used instead of estimating the balloon drift information. To consider the balloon drift information appropriately, the four-dimensional variational data assimilation (4DVAR) and a high horizontal resolution (6 km) are used. Cycling experiments over the observation period from 20 June to 4 July 2013 are carried out to obtain the statistical robustness of the effects of considering the balloon drift information, and a single-case experiment is also conducted to show further details about the effects. The verification results of cycling experiments, such as root-mean-square errors (RMSEs) for meteorological-variable forecasts verified against the radiosonde observations and threat and bias scores for rainfall forecasts, show the positive impacts of considering the balloon drift information. Results of the single-case experiment also reveal that the simulated rainfall distribution, time series of hourly rainfall, and quantitative precipitation forecast (QPF) skills are improved through the assimilation of radiosonde observations while considering the balloon drift information. Additionally, forecasts of meteorological variables such as horizontal wind components, temperature, and dewpoint temperature are also improved by considering the balloon drift information in the single-case experiment.