Background: cesarean section is the most common obstetric surgery in the world. Spinal anesthesia is a preferred technique for cesarean delivery for its distinct advantages over general anesthesia as the simplicity of the technique, reliability, minimal fetal exposure to drugs, patients' awareness and minimization of the hazards of aspiration. Aim of the Work: to define first time require analgesia in postoperative among two groups. Patients and Methods: this prospective comparative study was carried on fifty patients, ASA physical status II, aged from 18 to 45 years. These patients were scheduled for elective caesarean delivery under spinal anesthesia and divided to two groups. This protocol was approved by Research Ethical Committee of Ain Shams University. Written informed and verbal consent was obtained from each patient before being included in this procedure. Results:  the addition intrathecal 25mg pethidine make the total duration of analge­sia 169.20 ± 7.59 minute but adding 25μg fentanyl intrathecal extended the period of effective analgesia up to 178.40 ± 6.25 min with high significant P-value (0.000). The rapid onset of sensory and motor blocks in (F) group than (P) group with P value=0.000, also increased duration of sensory and motor blocks in (F) group than (P) group with P value 0.000. Conclusion: intrathecal opioid is a good technique of labor analgesia, although pethidine was the most widely used opioid for obstetric analgesia, it has character of local anesthetics so adding pethidine intrathecal in dose 25mg enhanced effect of local anesthetics but associated with more complications as nausea, vomiting and hypotension.