There have been several query-by-humming techniques developed for music retrieval. The techniques either are errorprone due to the inaccuracy of the hummed query or force the users to hum according to a metronome. This paper presents a new slope-based query-by-humming technique, in which the retrieval is robust to the inaccuracy in query and the use of metronome is eliminated. We use melody curve to represent the melodies of the original songs and the hummed query. And curve features: slope ranges and spans and note changes are extracted from the melody curves. Music retrieval is done by matching the curve features of query with those of the original musical songs. Results have shown the features and the algorithms are robust to humming inaccuracy.