Objective To evaluate the effect of dexmedetomidine added to ropivacaine for transversus abdominis plane block(TAPB) on postoperative pain in pediatric undergoing laparoscopic appendectomy. Methods Forty children(4~8 years,ASA 1-2) for Laparoscopic appendectomy were randomized to receive either an ultrasound-guided transversus abdominis plane block(TAPB) with plain ropivacaine 0.25% (Group R; n = 20) or ropivacaine 0.25% with dexmedetomidine1ug/kg (Group RD; n = 20) using a random number table. The TAPB was performed after surgery , and 0.4ml/kg of the study drugs was injected bilaterally. Bispectral index (BIS),Heart Rate(HR)and non-invasive Mean Arterial Pressure(MAP) were recorded at each time point before and after TAPB, and Improved Aldrete score and sedation (Ramsay) score were assessed in PACU. The length of stay in the PACU., T-pain free (the time interval of TAP block to the beginning of pain), immediate pain score FLACC, administration of supplemental analgesia and adverse events were assessed and recorded. Results The BIS at the time point T2, T3, and T4 in group RD was significantly lower than group R (P<0.05). The MAP at the time point T2 in group RD was higher than the group R (P<0.05). The HR did not differ between the study groups (P>0.05). The improved Aldrete score in group RD was significantly lower than group R,while Ramsay score was significantly higher than group R when the PACU was entered and out. The PACU stay in group RD was significantly longer than group R (P<0.05). T-pain free in group RD was significantly longer than group R(P<0.05). The FLACC of immediate pain score in group RD was significantly lower than group R(P<0.05). There is only one case for oral intake supplemental analgesia in group RD postoperatively. No adverse events were recorded in any of the study groups. Conclusions Dexmedetomidine added to ropivacaine for TAPB can significantly prolong the duration of painless postoperatively in pediatric undergoing laparoscopic appendectomy and enhance the efficacy of analgesia; But the postoperative sedative depth was increased and the length of stay in the PACU was prolonged.