Thank you all and I am sorry for the late reply. I had the same problem, Rohit Namjoshi. When I used Mesh -> {10,12,14,16,18}, there generated many lines. That's why I asked the help.

Thank you Hans Milton for the suggestion. I already tried ListContourPlot first. But the colour resolution was not good. That's why I tried to use ListDensityPlot. -_-a

I am still trying to find the answer....

Thanks for providing the code and data. Is this is what you are looking for? Not sure why there are more than 6 contour lines though

data = HeffTempStr[[All, {1, 2, 4}]];
ListDensityPlot[data,
 PlotLegends -> Automatic,
 AxesStyle -> Directive[Black, Thick],
 LabelStyle -> Directive[Black, Large, FontFamily -> "Helvetica"],
 ColorFunction -> "BlueGreenYellow",
 FrameTicks -> {{{10, 20, 30, 40, 50, 60}, None}, {{400, 500, 600}, None}},
 Mesh -> {10, 12, 14, 16, 18, 20},
 MeshFunctions -> {#3 &}]

Maybe ListContourPlot is a better option? If you hover over a contour line the corresponding z value is shown as a tooltip.

ListContourPlot[data,
 PlotLegends -> Automatic,
 AxesStyle -> Directive[Black, Thick],
 LabelStyle -> Directive[Black, Large, FontFamily -> "Helvetica"],
 ColorFunction -> "BlueGreenYellow",
 FrameTicks -> {{{10, 20, 30, 40, 50, 60}, None}, {{400, 500, 600}, None}},
 Contours -> {10, 12, 14, 16, 18, 20, 25}]

When I use Mesh -> {{10,12,14,16,18}}, it works. It's simple grammar. -_-a Thank you very much! Best regards, Jiwan.