Statement of Problem: Difficulties in sterilizing impressions by traditional methods have led to chemical disinfections as an alternative, and some studies have shown that some methods may adversely affect impressions.Purpose: This study evaluated the effectiveness of 5.25% sodium hypochlorite spray and immersion disinfection methods on irreversible hydrocolloid impressions (Iralgin alginate).Materials and Methods: In this study, twenty normal individuals were randomly selected. Impressions were made by a sterile tray from the maxillary arch and then were contaminated with the following organisms: staphylococcus aureus, streptococcus mutans, β-hemolytic streptococcus type A, Pseudomonas aeruginosa, Escherichia coli and Candida albicans. Impressions were cut and divided into three sections anterioposteriorly and longitudinally. Then, sections were randomly placed in three groups as 1) control (without disinfection), 2) immersion and 3) spray. Then, the provided samples (pieces) of impression materials were cultured and microbial count of the three groups were compared.Results: All of agar models from 20 alginate pieces in control group showed a positive microbial growth. Ten agar models form 20 alginate pieces which were disinfected by spraying method, showed also positive microbial growth and only 2 agar models out of 20 alginate pieces which were disinfected by immersion method showed a positive microbial growth. The mean number of organisms recovered form the surface of the impressions in control group was 3.52×108 CFU/ml while in spay and immersion groups it was 132217 CFU/ml and 1915 CFU/ml respectively.Conclusion: According to the results of this study, disinfection by immersion method with 5.25% sodium hypochlorite was more effective than the spray method.Key words: Disinfection, Alginate Impression, Sodium Hypochlorite