Introduction: It is important to keep a sufficient amount of circulating blood and tissue perfusion during surgery. Different compounds including crystalloid and colloidal solutions are proposed for this purpose. There is a lot of evidence that there is a clinically difference in the use of various crystalloids. In this study, the effect of two methods of fluid therapy with Ringer’ s Lactate and Ringer’ s solution on acid-base balance and postoperative complications in patients candidate for elective operation was studied. Subjects & Methods: This clinical trial study was performed on 86 patients undergoing major anesthetic surgery in ASA class I and II and had no heart and kidney problems. The fluid therapy was performed with a ringer's lactate solution in one group (n = 43) and in the other group with ringer solution (n = 43). Before starting surgery, blood samples were taken for hemoglobin, hematocrit, albumin and electrolytes, and arterial blood samples for analysis of arterial blood gas. At the end of the anesthesia, the patient was transferred to the recovery and was evaluated for recovery in the wake delay and other complications such as nausea and vomiting. Results: In this study, hemoglobin and hematocrit were significantly reduced in ringer's lactate groups after fluid therapy, and the amount of albumin after fluid therapy was reduced in the ringer's lactate group. There was no significant difference in sodium levels in both groups before and after fluid therapy. However, the amount of potassium and calcium significantly increased after the liquid therapy in the ringer's lactate group and the amount of chlorine in the ringer group significantly increased. Bicarbonate and PH were increased after fluid therapy in ringer's lactate group and decreased significantly in ringer group. Also, the amount of bleeding during operation and duration of postoperative hospitalization was higher in ringer group, but this difference was not statistically significant. Nausea and vomiting were reported more in the ringer group than in the ringer's lactate group, but this difference was not statistically significant. Also, the delay in waking in the ringer group was significantly higher than ringer's lactate group. Conclusion: According to the findings of this study, it seems that in elective surgery, the replacement of crystalloids ringer and ringer's lactate, there is a significant difference in PH and postoperative complications following the use of these serums. The blood acidity following fluid therapy with ringer was higher in comparison to fluid therapy with ringer's lactate, and the delay in awakening in this group was reported high.