In　this　paper,　a　robust　algorithm　to　detect　free-spaces　in　urban　roads　using　video　sequences　captured　on-board　is　proposed,　mainly　aiming　at　the　complex　urban　road　conditions　such　as　the　change　of　road　structure　and　dense　traffic　conditions,　as　well　as　the　changing　illumination.　This　algorithm　is　based　on　MSER　(Maximally　Stable　Extremal　Region)　extraction　using　gradient　images,　which　includes　the　following　two　aspects.　Firstly,　the　gradient　images　are　calculated　by　the　Sobel　operator　formed　as　the　spatial　filter　template.　MSERs　are　extracted　from　these　gradient　images　with　an　improved　MSER　extraction　algorithm　with　linear　complexity.　Then　some　shape　analysis　is　applied　on　each　MSER　after　shape　representation　is　computed　by　the　contour　extraction.　On　the　basis　of　selecting　the　proper　pixel　location　of　the　initial　seed　point,　the　region　of　the　road　area　is　determined　where　the　initial　seed　point　falls,　based　on　the　assumption　that　the　region　directly　in　front　of　the　host　vehicle　is　just　the　road　area.　Experimental　results　show　that　this　method　can　run　well　in　various　traffic　scenarios,　and　achieve　real-time　and　robust　road　detection　results.　©　2018　Technical　Committee　on　Control　Theory,　Chinese　Association　of　Automation.