In this study a comparison was set between the mean shear bond strength of resin composites to dentin related to maxillary teeth and that related to mandibular ones .Therefore the direction of application of an adhesive was being investigated .80 non – carious extracted human first molar (maxillary and mandibular ) were included in this study . They were divided into two groups of 40 to represent either the maxillary teeth or the mandibular ones. Each group was further subdivided into two subgroups of 20 to represent an immediate investigation or an investigation after one month. A universal bond adhesive ( BISCO – USA ), and the resin composite ( 3M FILTEK Z 250 XT ) were used in this study. The results showed that the mean shear bond strength of resin composite to dentin related to mandibular teeth was statistically significantly higher than that related to maxillary teeth. Also, the results showed that the incubation time ( one month ) had no significant effect on the mean shear bond strengths of all samples.