Objectives: Providing a good quality and safe medical service with medicines that are both effective and safe to patients is the main objective of any health care organization. This study aims to know the types and nature of prescribing errors (PEs), to check Drug - Drug Interactions (DDIs) occurrence and estimate rationality of prescription orders in a hospital with tertiary care. Materials and Methods: A cross sectional study of prescribing errors conducted in a private tertiary care hospital located in Jeddah city, Saudi Arabia over 3 months. One hundred thirteen patients were included in the study over the period of 12 weeks that was from October 2017 to December 2017. Results: It was detected in this study that as an average 4.35 drugs/prescription were prescribed. 24.8% of prescriptions found incomplete patient information. In this study we found that the majority of prescribing errors are not written the dose /dose error (7 %), lack of direction (2.7%) and also lack of duration of treatment (3.5%). Drug-drug interactions were analyzed by using Medscape drug interaction checker. Among one hundred thirteen prescriptions twenty seven prescriptions are found with minor drug-drug interactions (24%), nine prescriptions were identified with moderate interactions (8%) and two percent of prescriptions were observed with serious drug-drug interactions. Conclusion: In hospital setting computerizing the medication process system and providing the drug formulary in hospital may help the prescribers as quick reference for drug dose may help to lower prescribing errors. To curtail inappropriate application of drugs policy of drug use should be put into action.