Background: There have been many studies in using intrathecal morphine order to reduce pain after obstetrics and gynecological surgeries, abdominal surgeries and orthopedic surgeries. These studies have shown that intrathecal morphine is very effective for pain relief after surgery. However, intrathecal morphine also has side effects especially in obstetric and gynecological surgeries such as pruritus, postoperative nausea and vomiting, and delayed respiratory depression. Although postoperative analgesia with intrathecal morphine has been widely used in obstetrics and gynecological surgeries, orthopedic surgeries, there have been very few studies on postoperative pain relief with intrathecal morphine for colorectal surgery. Laparoscopic colorectal surgery requires multimodal analgesia, so using intrathecal morphine to reduce postoperative pain in this surgery is essential in clinical practice. Therefore, studying the effectiveness of intrathecal morphine in this surgery is necessary, so we conducted this study. Objectives: To assess the effectiveness of analgesic and side effects of intrathecal morphine after laparoscopic colorectal surgery. Materials and Methods: This was a descriptive, cross-sectional study, including 63 patients undergoing laparoscopic colorectal cancer surgery with intrathecal morphine before general anesthesia. The degree of analgesia was assessed based on VAS. The postoperative side effects observed were postoperative nausea and vomiting, respiratory depression, and pruritus. Results: The analgesic effect at rest and on slight movement was 95.2%, and 88.9% respectively with VAS ≤ 3. The side effects were postoperative nausea and vomiting (6.3%), and pruritus (1.6%), both postoperative nausea and vomiting and pruritus (3.2%). In conclusion, 300µg intrathecal morphine showed a safe and positive analgesic effect for laparoscopic colorectal cancer surgery.