how can i change fontsize and fontweight in xlabel and ylabel

how can i change fontsize and fontweight in xlabel and ylabel

What plotting package are you using? If you’re asking about Plots.jl, see guidefontsize from here:
http://docs.juliaplots.org/latest/attributes/
Posting a minimal working example would be a good idea too 
The Plots package does not seem to have an option for the font weight.
Quick&dirty solution using the PyPlot package:
using Plots
pyplot()
import PyPlot
rcParams = PyPlot.PyDict(PyPlot.matplotlib."rcParams")
font0 = Dict(
        "font.size" => 18,
        "axes.labelweight" => "bold",
        "axes.labelsize" => 16,
        "xtick.labelsize" => 8,
        "ytick.labelsize" => 8,
        "legend.fontsize" => 12,
)
merge!(rcParams, font0)
#############################################################
plt0 = plot(rand(10), rand(10), label="label 1", xlabel="x label", ylabel="y label", title="title")