We developed a methodology to estimate the canopy height from the ICESat/GLAS waveform for the purpose of contributing to the design of the Japanese spaceborne LiDAR mission; iss-jem LiDAR for Observation of Vegetation Environment (i-LOVE). We adopted an estimation method using a terrain index, which indicates the steepness of ground surface, to accurately estimate the canopy height in sloped areas. The study area is Hokkaido Island. We conducted a ground survey and collected airborne LiDAR data to use as the ground truth for the canopy height. We then developed some models to estimate the canopy height from a GLAS waveform. As a result, the estimation accuracy decreased in steep sloped areas where the terrain index exceeded 15 m. To reduce the influence of this effect, the estimation equation was separated for a gentle slope (terrain index ≤ 15 m) and a steep slope (terrain index 15 m). In this case, RMSE was 3 to 5 m. These findings indicated that an accurate estimation method would be ensured by using a footprint of less than 15 m of terrain index for the i-LOVE mission. On the assumption of a forested area located primarily at less than a 30° surface slope on a global scale, it is recommended that the diameter of the i-LOVE footprint should be less than 25 m. iLOVE is planned to transmit four laser pulses arranged at 2x2 simultaneously. This characteristic of i-LOVE, which does not require DEM, makes it possible to calculate the terrain index accurately and has a large advantage for accurately estimating the canopy height on a global scale.