How would you define , Lucretius views on pleasure, in his work, De Rerum Natura?
This is an excellent question. A few preliminary sentence will be important to answer this question.
First, it is correct to say that Lucretius was an Epicurean, which meant that pleasure or the absence of pain was one of the chief aims of this philosophy. In this sense, the work of Lucretius can be seen a hedonistic. However, there is a big qualification here. We must not be anachronistic. This leads me to my second point.
Second, for Lucretrius the way to gain pleasure was to live modestly and to understand the workings of the world. Lucretius believed that if a person understood that the world was made of atoms and that the gods were far off and did not really care about the everyday lives of humans, then people would live modestly and without fear. When this took place, then they would reach a state of tranquility (ataraxia). Here is a small quote that exemplifies this point from Lucretius "The greatest wealth is to live content with little, for there is never want where the mind is satisfied." This was the goal for Lucretius and other Epicureans.
Lucretius' massive epic poem on the Nature of Things is a manual for Epicurean living and an apologetic to defend the Epicurean worldview and to free people from the fear of death, so that they could live "pleasurable lives."