The purpose of this study was to determine whether different durations of conditioner application influenced glass ionomer cement penetration in dentin primary teeth. The conditioner being used was 10% poliacrylic acid. Samples in this study were 40 non-carious primary mandibular incisors. Samples were divided into 4 groups (10 samples each) : group A without conditioner, group B with seconds of conditioner application, group C with 20 seconds of conditioner application, and group D with 30 seconds of conditioner application. penetration of glass ionomer cement was observed using scanning electron microscopy (SEM) with 2000X magnification. One-way ANOVA and Tukey HSD test showed significant difference between groups. Longer conditioner application resulted in longer glass ionomer penetration in dentin of primary teeth.