A set of precision, vertical, white-beam slits has been designed for an undulator/wiggler beamline at the Advanced Photon Source (APS). The slit, a knife-edge-type precision device, is required to have very small thermal distortion during operation with beam. The traditional slit consists of a cooling block and an OFHC cooling channel inside the block. Our design consists of one large block and an OFHC cooling tube (filled with copper mesh) brazed inside the large block. This design will accommodate the x-ray source from both undulators and wigglers. Due to the powerful x-ray heat flux coming from APS Undulator A, it is an exceedingly difficult problem to reduce the thermal distortion to less than 50 microns as required by some users.