This paper presents a spreadsheet-based reliability prediction simulation framework for the conceptual product design stage to acquire system reliability information in timely manner. During early stage, reliability performance deals with both known and unknown failure rates and component-level and subsystem-level failure estimate to predict system reliability. A technique for performing Monte-Carlo simulation using Excel spreadsheet has been developed under the such circumstances. The Functional Reliability Block Diagram (RBD) of the system under investigation is first transformed into a table in an Excel Spreadsheet. Each cell within the table corresponds to a specific block in the RBD. The Excel pseudo random number generator is used to simulate failure times of individual components or subsystems in the system. Excel function formulars for failure times entered into these cells are in accordance with the failure time distribution of the corresponding block. For unknown component failure rates, triangular probability distributions are used. Logical expressions are then used to determine system success or failure. Excel`s macro feature enables repetition of the scenario thousands of times while automatically recording the failure data. Excel`s easy graphical capabilities are later used for plotting the diverse reliability performance measures such as PDF and CDF of the overall system. This paper also discuss the results obtainable from this method such as reliability estimate, mean and variance of failures and confidence intervals. The resultant of this reliability prediction system is mainly benefitting small and medium-sized enterprise`s field engineers.