It is known that fluid-borne noise emitted from a hydraulic system often causes severe discomfort of the operator. It is also known that the noise can change as the system warms up. The temporal change in steady operation can be a factor for misperception of mechanical failure, because noise can play an important role as a signal that indicates abnormal operation. It is therefore important to understand the behavior of hydraulic pressure ripples that are a source of hydraulic noise in steady operation, and how they change in relation to the operating time. This study has investigated the influence of oil temperature changing over time on pressure ripples in a hydraulic line in steady operation. Variables that can change in steady operation have been considered using mathematical models of the pressure ripples. The consideration has shown that, when there is no air-entrainment from an outer system, the change over time is caused solely by the change of the hydraulic oil temperature. Physical properties of the oil and speed of sound in the oil have been defined as temperature-related variables in the mathematical models. The physical properties that are used in the mathematical models have been obtained directly from the manufacturer. In contrast, the speed of sound in the oil has to be obtained using theoretical approaches. The behaviors of the hydraulic pressure ripples in three simple hydraulic systems have been investigated using mathematical models that took into account the thermal properties. Each hydraulic system consists of a hydraulic pump, a straight pipe and a termination condition that is one of the following typical three types: non-reflection, valve and volume. The mathematical models have shown distinct behaviors resulting from the change in the oil temperature. The behaviors have been verified experimentally.