The purpose of this paper is to investigate the effect of the different strengthening techniques on the behavior of continuous one-way slab with insufficient length of development of rebars at the internal joint. The insufficient length of development of rebars in the zone of negative steel reinforcement leads to a reduction in force which has dangerous consequences that affect the structure and its serviceability. This study proposed three strengthening techniques to compensate the insufficient development length which including: internal strengthening by confining the lap zone by spiral bar, externally strengthened by CFRP sheet and steel plates. Six one-way concrete slabs with dimensions of 2200 mm length, 500 mm width, and 125 mm thickness were casted and tested under static load. The main variables adopted in this study are: the length of development, type of strengthening, and presence of construction joint. All slabs were loaded to failure. Form results, it’s found that the presence of construction joint and insufficient length of development of rebars at internal support of the continuous one-way slab had a tangible influence on the structural behavior, also, the insufficient length of development resulted in a deterioration of the structure, decrease in stiffness, and change in the mode failure. It was observed that the strengthening techniques used in this study able to compensate the reduction in stiffness and enhancing mode of failure. All the proposed strengthening techniques was efficient especially steel plates which gave a high increasing in ultimate load by about 16.14 % and by about 8.7-3.73% for the other.