Statement of the Problem:Although sumac extract (SE) is reported as a collagen cross linker, the available data regarding its effect on the dentine micro-hardness is quite sparse.Purpose:Therefore, the aim of this study includes evaluating the effect of different concentrations of SE on dentine micro-hardness comparing to grape seed extract (GSE).Materials and Method:In this experimental study, the GSE was purchased from available market and convert to 5% solution. Meanwhile the 5, 10, and 20% of SE solutions were prepared experimentally. The base line micro-hardness of 60 samples (30 premolars divided to buccal and lingual segments) was recorded triplicate for each specimen and they were randomly divided into 5 groups (four abovementioned experimental solutions and de-ionized water as negative control). For 35 consecutive days, each sample was twice pH cycled and treated by solutions. Ultimately, the final micro-hardness was recorded triplicate again for each sample and the numerical data was compared with each other using one-way ANOVA and Tukey HSD Post Hoc tests (α=0.05).Results:The meanSD values of micro-hardness for the groups was recorded as 54.45 13.4, 65.6518.5, 39.572.26, 41.131.66 and 43.794.96 at base line and 10.40.99, 11.85 0.75, 10.161.84, 8.481.16 and 6.311.01 at final stage for control, GSE 5%, SE 5%, SE 10% and SE 20% respectively. There was no significant difference among the micro-hardness of the groups before treatment (p= 0.369). However, after experimental treatment, there was significant difference between the groups (p= 0.024) while in pairwise comparison just two groups (GSE 5% and SE 20%) had significant difference with each other (p= 0.017).Conclusion:The efficacy of SE was reversely related to its concentration. Moreover, neither GSE nor SE had significant effect on dentine micro-hardness after 35 day pH cycling.