Microbial contaminations in the culture plates are quite common in microbiological laboratory. The preparation of culture plates is an art thereby the scientific principles should be adhered strictly. While preparing the culture plates, chance of contamination is high either due to inadequate sterilization or improper handling. In general, the prepared plates should not use or store at low temperature immediately. The sterility checking is very important before impregnation the plates for microbiological procedures. This study is aimed to analyze the discarding criteria of the contaminated plates either partly or completely. Two sets of nutrient agar plates were prepared and kept for sterility checking overnight at room temperature. The type of contaminants were assessed; thereby colonies on the surface or underneath the surface. Both the sets were kept for another day for the development of contamination in the contaminant free plates recorded on the day 1. The results revealed that two types of contaminants were found in two sets. Set one have the colonies on the surface and no further contamination found on day 2 in the non-contaminated plates indicated that the contamination may occur during plating procedures. Set two have the colonies underneath the surface and contamination developed on day 2 in the non-contaminated plates due to improper sterilization of the glassware and culture media. Thus from this study, it was recommended that the set one plates which are not contaminated are used for further processing, whereas the entire set two should be discarded. The major significance of this work is to understand the importance of sterility checking before using the nutrient plates for culturing.