Silicone-based tissue-mimicking phantom is widely used as a surrogate of tissue for clinical simulators, allowing clinicians to practice medical procedures and researchers to study the performance of medical devices. This study investigates using the mineral oil in room-temperature vulcanizing silicone to create the desired mechanical properties and needle insertion characteristics of a tissue-mimicking phantom. Silicone samples mixed with 0, 20, 30, and 40 wt. % mineral oil were fabricated for indentation and needle insertion tests and compared to four types of porcine tissues (liver, muscle with the fiber perpendicular or parallel to the needle, and fat). The results demonstrated that the elastic modulus and needle insertion force of the phantom both decrease with an increasing concentration of mineral oil. Use of the mineral oil in silicone could effectively tailor the elastic modulus and needle insertion force to mimic the soft tissue. The silicone mixed with 40 wt. % mineral oil was found to be the best tissue-mimicking phantom and can be utilized for needle-based medical procedures.