Although the stress analysis of the adhesively bonded single lap joints(SLJ) has been studied in many papers there are limited investigations upon stiffness prediction of the adhesively bonded joints. There is also no study on the viscoelasticity effect on stiffness prediction of the SLJ in the open literature. In this study an analytical model was proposed for the stiffness prediction of the bi-adhesive SLJ with use of two types of adhesives SikaFast 5215 NT and SikaPower 4720 as flexible and stiff adhesives, respectively. Bond-length ratio as a definition of the ratio of flexible adhesive length to stiff adhesive length was taken as d=0.2, 0.5, 1 and 2. Bi-adhesive and mono adhesive results were compared. It was concluded that the joint stiffness can be strongly affected by the stiff adhesive proportion of the overlap length. Due to time dependent elasticity modulus as a result of viscoelastic properties of adhesives, joint stiffness reduces while time progresses. Results showed that adhesive stresses and joint stiffness can be optimized by using bi-adhesive joint with an appropriate bond-length ratio. The adherend material effect was also studied. It was concluded that the adherend material strongly affects the stiffness of the joint.