An in situ observation system composed of a latticework, a mirror, and a video camera has been developed to quantitatively determine the transient deformation of wafers when they are pushed into or pulled from a hot horizontal furnace. To observe a silicon wafer in the middle of many silicon wafers on a boat, those wafers that are located in front of the silicon wafer we want to observe are replaced by an equal number of same-dimension transparent quartz wafers. An approximate relation for obtaining the actual amount of wafer distortion from the experimentally observed image distortion is also derived. Using this observation system, we examined in detail the effects of furnace temperature, boat speed, wafer location, and wafer thickness on transient deformation. This system also provides more complete information on deformation distribution across a wafer, deformation speed, and recovery speed.