Image segmentation technique for Ziehl-Neelsen sputum slide image is an important factor in extracting tuberculosis (TB) bacilli from the background and sputum cells. Colour threshold using Red, Green, Blue (RGB) information has been used widely in many researches. This paper provides new approach to perform thresholding using Hue, Saturation, and Intensity (HSI) information. Performance comparison between HSI and RGB thresholding methods were carried out for Ziehl-Neelsen sputum slide images. Five types of different images have been used to compare the performance of HSI and RGB thresholding methods. The result shows that thresholding using HSI information gives better result compared to RGB.